APTA STANDARDS DEVELOPMENT PROGRAM
RECOMMENDED PRACTICE
American Public Transportation Association
1300 I Street, NW, Suite 1200 East, Washington, DC 20006
APTA BTS-BC-RP-010-20
Published: February 4, 2020
Bus Brake & Chassis Working Group
This document represents a common viewpoint of those parties concerned with its provisions, namely transit
operating/planning agencies, manufacturers, consultants, engineers and general interest groups. The application of
any recommended practices or guidelines contained herein is voluntary. APTA standards are mandatory to the extent
incorporated by an applicable statute or regulation. In some cases, federal and/or state regulations govern portions of
a transit system’s operations. In cases where this is a conflict or contradiction between an applicable law or regulation
and this document, consult with a legal advisor to determine which document takes precedence.
© 2020 The North American Transportation Services Association (NATSA) and its parent organization APTA. No part of this
publication may be reproduced in any form, in an electronic retrieval system or otherwise, without prior written permission of NATSA.
Troubleshooting Transit Bus Air
Systems
Abstract: This recommended practice provides guidelines for troubleshooting transit bus air systems,
including basic design criteria, preventive maintenance and common problems.
Keywords: accessory system, air system, emergency brake, parking brake, service brake system, supply
system, valves
Summary: This document establishes a recommended practice for troubleshooting transit bus air systems.
Individual operating agencies should modify these guidelines to accommodate their specific equipment and
mode of operation. The following recommended practices and guidelines assume that the end users have
sufficient skills and knowledge to repair and maintain the related systems at a journeyman level. These skills
and knowledge must also include a fluent understanding of safe shop working practices, not only for the
agency but also OSHA/CCOHS/provincial/federal/state and local safety standards. A familiarity with
applicable industries, component/system suppliers and vehicle manufacturers is also assumed.
Scope and purpose: Not all air systems are included in this document, and the tables and examples it
contains are commonly used for transit applications. The purpose of this recommended practice is to provide a
uniform method for air system troubleshooting in order to restore brake performance.
© 2020 American Public Transportation Association | ii
Table of Contents
Participants ......................................................................................................................................................... iii
Introduction ........................................................................................................................................................ iii
1. Safety provisions .......................................................................................................................................... 1
2. Opening statement ........................................................................................................................................ 1
3. Supply system ............................................................................................................................................... 2
3.1 Overview ....................................................................................................................................................... 2
3.2 Troubleshooting ............................................................................................................................................ 5
4. Service (control) brake system .................................................................................................................. 10
4.1 Overview ..................................................................................................................................................... 10
4.2 Troubleshooting .......................................................................................................................................... 12
5. Parking brake and emergency brake system ........................................................................................... 13
5.1 Overview ..................................................................................................................................................... 13
5.2 Troubleshooting .......................................................................................................................................... 14
6. Accessory system ....................................................................................................................................... 17
6.1 Overview ..................................................................................................................................................... 17
6.2 Troubleshooting .......................................................................................................................................... 17
Related APTA standards ................................................................................................................................... 20
Abbreviations and acronyms ............................................................................................................................. 20
Document history .............................................................................................................................................. 20
List of Figures and Tables
FIGURE 1 Typical Air Brake System .......................................................... 2
FIGURE 2 Supply System Components ....................................................... 3
TABLE 1 Air Supply System Troubleshooting Guide ................................. 5
FIGURE 3 Service (Control) Brake System Components .......................... 11
TABLE 2 Service (Control) Brake System Troubleshooting Guide ........... 12
FIGURE 4 Parking and Emergency Brake System Components ............... 14
TABLE 3 Parking and Emergency Brake System Troubleshooting Guide 14
TABLE 4 Accessory System Troubleshooting Guide ................................ 17
© 2020 American Public Transportation Association | iii
Participants
The American Public Transportation Association greatly appreciates the contributions of the Bus Brake &
Chassis Working Group
, which provided the primary effort in the drafting of this document.
At the time this standard was completed, the working group included the following members:
Jerry Guaracino, Chair
James Baldwin
Mark Barker, Haldex Brake Products
Tom Baurmann, MAN Engines & Components
Kenneth Bisson, Greater Cleveland RTA
Pat Breen, SEPTA
John Brundage, Jacobs
John Campo, Power Brake
Bruce Dahl
Garrett Davis, Webb Wheel Products
Carlos Manuel Delgado, Miami-Dade Transit
Tim Derr, MAN Engines & Components
David Domine, Link Engineering Company
Richard Dooley, Central Ohio Transit Authority
Joe Doyle, Marathon Brake Systems
Heiner Falke, MAN Engines & Components
Steve Farrar, Bendix
Mitch Forbes, Haldex Brake Products
Frank Forde, Los Angeles County Metropolitan
Transportation Authority
Jim Fox, Charlotte Area Transit System
Victor Guillot, WMATA
Samet Gursel, Maryland Transit Administration
Jim Heuchert, New Flyer Service Organization
Chip Hurst, Webb Wheel Products
Randy King, MGM Brakes
Michael Konrad, Bremskerl North America
David Kwapis, MBTA
David Lawrence, Fraser Gauge
Geoff Lawrence, Fraser Gauge
Ricky Mares, Harris County METRO
Brian Markey, Custom Training Aids
Dennis McNichol, Link Engineering Company
Peter Morse, Commercial Vehicle Components
Kenneth Peterson, King County Metro
Karl Robinson, NFI Parts
Christopher Sabol, Haldex Brake Products
James Szudy, Bendix
Don Tirrell, MGM Brakes
Oscar Tostado, OMNITRANS
Anthony Van de Riet, Bi-State Development
Agency
Hans Wimmer, Friedrichshafen AG
John Wolf, Meritor
Aaron Woods, ABC Companies
Project team
Lisa Jerram, American Public Transportation Association
Bruce Dahl, Contractor
Introduction
This introduction is not part of APTA BTS-BC-RP-010-19, “Troubleshooting Transit Bus Air Systems.”
APTA recommends the use of this document by:
individuals or organizations that operate bus transit systems;
individuals or organizations that contract with others for the operation of bus transit systems; and
individuals or organizations that influence how bus transit systems are operated (including but not
limited to consultants, designers and contractors).
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 1
Troubleshooting Transit Bus Air Systems
1. Safety provisions
WARNING: Failure to comply with the safety provisions in this document can result in personal
injury or death.
2. Opening statement
Proper maintenance will ensure the safe and dependable operation of a transit vehicle. Buses should be
maintained to comply with OEM maintenance guidelines, as well as federal, state, provincial and local codes
and regulations.
This document is designed to support four air subsystems consisting of the following:
supply
service brake
parking and emergency brake
accessory
This document contains an overview and troubleshooting charts that describe symptoms, possible causes and
corrective actions.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 2
3. Supply system
FIGURE 1
Typical Air Brake System
3.1 Overview
WARNING: Alterations to vehicle air systems can lead to personal injury or death and affect
the safe operation of the vehicle. No alterations shall be made that will compromise the
design intent of the OEM. Replacement parts should meet OEM specifications.
The supply system (Figure 2) is designed to provide an adequate supply of clean, dry air to meet the needs of
the following air subsystems:
primary service brake
secondary service brake
parking and emergency brake
accessories
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 3
FIGURE 2
Supply System Components
There are two common types of compressor systems: engine-driven reciprocating compressors and electric
scroll compressors:
Engine-driven reciprocating compressors are typically driven by the engine’s front accessory
drive. They typically bring air in from the engine’s air inlet filter system. They may either be
naturally aspirated or turbo-aspirated.
• In naturally aspirated installations, the air is taken from ahead of the engine turbocharger
system, at slightly below atmospheric pressure.
• In turbo-aspirated installations, the air is taken from the engine’s inlet manifold after the
turbocharger, where it has already been raised somewhat above atmospheric pressure by the
engine’s turbocharger. Turbo-aspirated installations are typically preferred, as the positive
inlet pressure helps reduce oil passing from the compressor’s cylinders, while also increasing
the efficiency of the air delivery performance. However, the air delivery temperature can be
higher than naturally aspirated installations and are subject to oil and debris in the event of a
turbocharger failure.
Electric scroll compressors are common on hybrid, trolley or battery-electric buses. They typically
have an air inlet/filtration system separate from the engine and are typically naturally aspirated. They
are electrically driven at high voltage (230 or 460 V three-phase AC), either direct-drive or belt
driven, and controlled by an electrical contractor system through the multiplex system. Some use oil-
free scroll compressors, while others use oil-lubricated vane compressors. Vane compressors add
lubricating oil to the air flow to lubricate the vanes, and therefore must use oil separators to remove
the oil after compression and prior to moving to the reservoir system. Some systems also incorporate
after-coolers integrated into the compressor package
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 4
The filtered air is compressed and delivered to the supply reservoir through an air dryer. Air pressure is
controlled by the governor. The air dryer removes moisture/oil and other contaminants. The supply reservoir
air is protected by a one-way check valve (typically located in the air dryer). The supply system is typically
protected by multiple pressure relief/safety valves. Usually a 250 psi safety valve is located on or in the air
compressor, a 175 to 200 psi safety valve is located at the air dryer, and a 150 psi safety valve is installed in
the supply reservoir.
If the supply reservoir pressure is controlled by a mechanical air governor, then the reservoir port on the
governor receives a pressure signal from the supply reservoir. When air pressure falls below the governor
cut-in setting (commonly between 100 and 120 psi), the air compression begins. When the reservoir pressure
reaches the governor cut-out setting (typically 120 to 135 psi), air compression ceases and the air dryer
purges.
NOTE: Some dryers may use other means to initiate purging.
Electrically driven air compressors may use an electronic control system. These systems would use air
pressure transducers located in the reservoir system to monitor reservoir pressure and use these signals, in
conjunction with the PLC system, to engage or disengage the compressor drive motor.
All reservoirs are required to include a manual drain valve. Some reservoirs may also be equipped with an
automatic drain valve.
There are several important performance criteria that can be determine with troubleshooting the items in
Table 1. They include the following:
Air pressure buildup time: There are a variety of test procedures to measure buildup time. The
recommended method is to build up air pressure to the governor cut-out pressure and then, by
pumping the brakes, to reduce gauge pressure to less than 85 psi. With engine at fast idle
(approximately 1000 RPM), the time required for air pressure to build from 85 to 125 psi should be
within 45 seconds. Refer to the OEM in-service standards and/or local DOT for each vehicle.
Discharge line temperatures: Discharge line temperature is measured at compressor outlet fitting
and the dryer inlet fitting under maximum compressor load. The typical compressor outlet
temperature should be less than 350 °F. The typical air dryer inlet temperature should be less than
160 °F.
NOTE: OEM specifications may vary.
Static air loss: Static air loss is measured with the air system built up to governor cut-out pressure,
the engine not running, service brakes released and the parking brake applied. The maximum air loss
for a transit bus is 2 psi per minute.
Applied air loss: Applied air loss is measured with the air system built up to governor cut-out
pressure, the engine not running, the service brake fully applied and the parking brake released. The
maximum applied air loss for a transit bus is 3 psi per minute.
Air compressor duty cycle: Air compressor duty cycle is the percentage of time the air compressor
is compressing air over a predetermined period. Duty cycles on reciprocating compressors should not
exceed 30 percent, and on electrically driven scroll compressors, duty cycle should not exceed
70 percent. Duty cycles above these limits are considered excessive and may require more frequent
maintenance and/or repair.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 5
3.2 Troubleshooting
TABLE 1
Air Supply System Troubleshooting Guide
Symptom: Oil or air leaking at compressor connections
Suggested Action(s)
If fitting is loose, re-torque or replace the fitting gasket/O-ring and then
re-torque. Inspect inlet hose or hard lines and replace as necessary.
If fitting is loose, re-torque or replace gasket/O-ring or fitting as necessary
to ensure a good seal.
Check that fitting is secured, and check the hose for damage. Repair or
replace as necessary.
vane compressor)
Check oil level of compressor. Verify proper functionality of oil separator.
Symptom: Oil leaking from compressor head and/or crankcase
(Note: A small amount of weeping from head gasket is acceptable).
Suggested Action(s)
Check for restriction in discharge line and oil return line. Replace or
repair as necessary. After the source of an “excessive” leak has been
repaired, the gasket and head bolts or compressor should be replaced.
Clean and inspect sealing surfaces. Reseal cover plate with RTV sealant
and re-torque. Apply thread sealant on a new sump plug, and install with
the appropriate tool(s).
Clean off the compressor and monitor periodically to isolate the source of
the leak.
Symptom: Fluids in the supply or service reservoir(s)
Suggested Action(s)
Check for air leaks. Perform leak-down test using soapy water on air
subsystems. Isolate and repair leaks.
Repair/replace compressor as needed.
inlet temperature too high
Check discharge fitting temperature. If compressor discharge
temperature exceeds 350 °F and/or dryer inlet temperature exceeds
160 °F, then check carbon buildup in compressor exhaust fitting/line and
the air dryer inlet fitting and filter. Repair or replace as needed.
Inspect coolant lines and fittings for kinks, accumulated rust scale and
restrictions. Consult engine and/or compressor manufacturers for specific
information on testing and acceptable coolant flow rates and
temperature. Replace or reroute lines as necessary.
Check coalescent filter and replace if needed. Check discharge line for
carbon buildup. If build-up thickness is greater than
1
∕
16
in., then replace
discharge line. Be sure the discharge line maintains a constant slope
downward to the muffler tank and/or air dryer to prevent blockage due to
freezing.
Check compressor air inlet line for restrictions (not to exceed 30 in. of
water), brittleness or soft or sagging hose conditions. Repair as
necessary. Check the engine air filter, and service if necessary.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 6
TABLE 1
Air Supply System Troubleshooting Guide
Inspect control lines to and from the governor for air leaks and
restrictions. Use soapy water to check for air leaks, and use a calibrated
external gauge in the supply or service reservoir to verify that cut-in and
cut-out pressures are within vehicle OEM specifications. Replace or
adjust governor as necessary.
Check for governor malfunction, excessive air leak, excessive duty cycle
(insufficient purge time), restricted line from external purge tank,
contaminated or saturated desiccant.
overdue maintenance
Verify operation of air dryer. Check for carbon buildup in dryer inlet fitting
and port. Check that the dryer inlet filter is properly cleaned during the
purge cycle. Follow vehicle OEM maintenance recommendations and
service data information. Clean and replace if necessary.
Check the engine oil pressure with a test gauge and compare with
manufacturer specifications. Consult manufacturer for specific
recommendations on troubleshooting.
Check for excessive duty cycle, restricted coolant flow to compressor,
dirty or plugged aftercooler or copper discharge tube.
Test for excessive engine crankcase pressure. A common indicator of
excessive crankcase pressure is a loose or partially lifted dipstick. Repair
or replace crankcase ventilation components as necessary.
Check for excessive bends, kinks or other restrictions in the external oil
return line. Repair, replace or reroute as necessary.
Replace the compressor when the above possible causes have been
ruled out.
Symptom: Air system is slow to build pressure
Suggested Action(s)
Perform leak-down test on air subsystems. Listen for audible source or
use soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair
as necessary.
Follow compressor manufacturer recommendations for determining
proper operation of compressor unloader.
(reciprocating compressor)
An excessive leak around the head gasket may be caused by flow
restriction downstream, a dead-headed compressor, malfunctioning
governor or a defective/missing safety valve. Check for restrictions or
faulty safety valve, and repair as necessary. Replace compressor.
Check discharge line for carbon buildup. If buildup thickness is greater
than
1
∕
16
in., then replace discharge line. Be sure the discharge line
maintains a constant slope downward to the muffler tank and/or air dryer
to prevent blockage due to freezing.
Check compressor air inlet line for restrictions, (not to exceed 30 in. of
water) brittleness, or soft or sagging hose conditions. Repair as
necessary. Check the engine air filter and service if necessary.
Replace the compressor when the above possible causes have been
ruled out.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 7
TABLE 1
Air Supply System Troubleshooting Guide
Symptom: Air system does not build air pressure
Suggested Action(s)
Inspect control lines to and from the governor for air leaks and
restrictions. Use soapy water to check for air leaks, and use a calibrated
external gauge in the supply or service reservoir to verify that cut-in and
cut-out pressures are within vehicle OEM specifications. Replace or
adjust governor as necessary.
Check PLC control system for proper operation. Check high-voltage
supply and switching systems.
Check discharge line for carbon buildup. If buildup thickness is greater
than
1
∕
16
in., then replace discharge line. Be sure the discharge line
maintains a constant slope downward to the muffler tank and/or air dryer
to prevent blockage due to freezing.
Remove water from system and eliminate the discharge line dip by
improving the slope. After repair, verify proper compressor function.
Refer to OEM recommendations for testing, and make necessary repairs.
Check for damaged discharge line, loose connections, safety or
discharge valve stuck open, muffler (ping) tank malfunction.
Replace the compressor when the above possible causes have been
ruled out.
Symptom: Compressor safety valve releases air
Suggested Action(s)
Check coalescent filter and replace if needed. Check discharge line for
carbon buildup. If buildup thickness is greater than
1
∕
16
in., then replace
discharge line. Be sure the discharge line maintains a constant slope
downward to the muffler tank and/or air dryer to prevent blockage due to
freezing.
Inspect air lines, and verify that check valves are working properly.
Repair or replace as needed.
Ensure that discharge line is installed into the air dryer inlet and that
delivery is routed to the supply/service reservoir.
Replace as necessary. Verify that relief pressure is consistent with
vehicle OEM specifications.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 8
TABLE 1
Air Supply System Troubleshooting Guide
Symptom: Air dryer safety valve releases air
Suggested Action(s)
Inspect delivery lines to reservoir for restrictions, and repair as
necessary.
Follow compressor manufacturer recommendations for determining
proper operation of compressor unloader.
Confirm specification with OEM.
Replace as necessary.
Verify operation of air dryer. Follow vehicle OEM maintenance
recommendations and service data information.
reservoir
Ensure that governor control line from the reservoir is located at the top
of the reservoir. Lines located at the bottom of the reservoir can become
blocked or restricted by contaminants.
out
Inspect control lines to and from the governor for restrictions and air
leaks. Use soapy water to check for leaks, and use a calibrated external
gauge in the supply or service reservoir to verify that cut-in and cut-out
pressures are within vehicle OEM specifications. Replace or adjust
governor as necessary.
Install correct safety valve per vehicle manufacturer’s specifications.
Symptom: Reservoir safety valve releases air
Suggested Action(s)
Follow compressor manufacturer recommendations for determining
proper operation of compressor unloader.
pressure setting for application.
The release pressure of a given valve is stamped into the side of the
valve. Verify that relief pressure is at vehicle OEM or component
manufacturer specifications. Connect a calibrated external pressure
gauge to the service reservoir, and verify system pressure. Replace as
necessary.
out
Inspect control lines to and from the governor for restrictions and air
leaks. Use soapy water to check for leaks, and use a calibrated external
gauge in the supply or service reservoir to verify that cut-in and cut-out
pressures are within vehicle OEM specifications. Replace or adjust
governor as necessary.
Diagnose and repair.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 9
TABLE 1
Air Supply System Troubleshooting Guide
Symptom: Air dryer does not purge
Suggested Action(s)
Repair or replace dryer purge valve. Check purge valve control circuit.
Repair or replace purge solenoid valve. Check electric purge valve
control circuit.
Verify operation of air dryer.
• Single tower type: purge valve
• Alternating tower type: shuttle valves, purge timer
Follow vehicle OEM maintenance recommendations and service data
information.
Remove plug or clear restriction
Inspect control lines to and from the governor for restrictions or a plug
installed in delivery port to compressor unloader. (Verify proper air line
installation.) Use a calibrated external gauge in the supply or service
reservoir to verify that cut-in and cut-out pressures are within vehicle
OEM specifications. Replace or adjust governor as necessary.
leakage or excessive air usage
Perform leak-down test on air subsystems. Listen for audible source, or
use soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair
as necessary.
reservoir
Ensure that the governor control line from the reservoir is located at the
top of the reservoir. Lines located at the bottom of the reservoir can
become blocked or restricted by contaminants. Check that reservoir and
unloader lines are not reversed.
Symptom: Compressor constantly cycles
Suggested Action(s)
performed
Available reservoir capacity may be reduced by buildup of contaminants
in tank reservoirs. Drain and perform routine maintenance per vehicle
OEM or component manufacturer service data.
Check and remove any restrictions in exhaust port.
Follow compressor manufacturer recommendations for determining
proper operation of compressor unloader.
Verify operation of air dryer. Follow vehicle OEM maintenance
recommendations and service data information.
Perform leak-down test on air subsystems. Listen for audible source, or
use soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair
as necessary.
Inspect control lines to and from the governor for air leaks and
restrictions. Use soapy water to check for air leaks, and use a calibrated
external gauge in the supply or service reservoir to verify that cut-in and
cut-out pressures are within vehicle OEM specifications. Replace or
adjust governor as necessary.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 10
TABLE 1
Air Supply System Troubleshooting Guide
Symptom: Compressor leaks air
Suggested Action(s)
Check for leaking, damaged or defective compressor fittings, gaskets,
etc. Repair or replace as necessary.
Repair or replace as necessary. Verify that relief pressure is consistent
with vehicle OEM specifications.
An air leak at the head gasket may indicate a flow restriction downstream
of the compressor or a defective/missing safety valve. Check for
restrictions or a faulty safety valve. Replace or repair compressor.
fittings
Inspect for loose or over-torqued fittings. Reseal and tighten loose fittings
and plugs as necessary. If over-torqued fittings and plugs have cracked
ports in the head, then the compressor will need to be replaced.
Symptom: Compressor leaks coolant (external)
Suggested Action(s)
A leaking head gasket may indicate a flow restriction downstream of the
compressor or a defective/missing safety valve. Check for restrictions or
a faulty safety valve. Check for correct coolant flow. Check for air
blockage in discharge line and air dryer. Once the source of the problem
has been repaired, replace head or compressor.
Inspect for loose or over-torqued fittings. Reseal and tighten loose fittings
and plugs as necessary. If over-torqued fittings and plugs have cracked
ports in the head, then the compressor will need to be replaced
Symptom: Compressor pressurizes engine coolant system
Suggested Action(s)
If casting porosity is detected, then replace head or compressor.
Replace head or compressor.
Check for downstream air blockage in the discharge line and air dryer.
Make necessary repairs, and replace head or compressor.
4. Service (control) brake system
4.1 Overview
All vehicles on the road require a service (or control) braking system; see Figure 3. Simply stated, when the
driver steps on the brake pedal, the vehicle will slow or stop in direct proportion to the amount of treadle
pedal force exerted. Equally important to the vehicle control afforded by a brake application is the driver’s
ability to release the brakes when necessary. Therefore, the entirety of the brake’s service system involves
controlled brake pedal application and release. Since the majority of normal vehicle braking involves gradual
slowing, as opposed to panic stopping, the driver must be able to make controlled hard or soft stops as
required. This correlation between treadle pedal force and stopping power is called “modulation.”
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 11
FIGURE 3
Service (Control) Brake System Components
In an air brake system, service brakes are pneumatically powered. The brake pedal, when pressed,
mechanically operates the brake (treadle) valve, which in turn delivers air pressure to brake chambers
(actuators) that convert air pressure into linear mechanical force. When the pedal is released, air pressure from
the chambers is released to the atmosphere, thereby releasing the mechanical force from the foundation brake
components.
In addition to the brake (treadle) valve and chambers, stop light switches, relay valves and quick-release
valves are employed in the service brake system. During a service brake application, air pressure is delivered
to the stop light switch, which completes an electrical circuit to apply the brake lights. A quick-release valve
is typically used to distribute delivery air pressure to each front brake chamber and quickly release air,
without it having to travel back to the treadle valve. The relay valve located near the rear axle speeds up the
application and release of the rear brakes. It has a dedicated supply of air from the primary reservoir,
requiring only an air control signal from the treadle valve. Quick-release valves can also be used in the control
circuit to speed application and release of relay valve control signals.
In the interest of safety through redundancy, service brakes have independent rear (primary) and front
(secondary) air circuits. In the event of an air loss in one of these circuits, the remaining circuit will still
function to permit vehicle braking via the treadle valve.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 12
4.2 Troubleshooting
TABLE 2
Service (Control) Brake System Troubleshooting Guide
Symptom: Poor brake modulation; cannot make gradual brake application
Suggested Action(s)
sticking
Inspect linkage, treadle pin, roller, boot and piston. Look for corrosion
and/or contamination. Replace or repair as necessary. (Reference APTA
recommended practice “Transit Bus Brake Valve Treadle Assembly
Maintenance”.)
Repair or replace brake (treadle) valve.
If brake (treadle) valve is functioning properly, then check downstream
delivery lines for uniform pressure modulation. Replace damaged or
restricted lines.
If modulated air pressure is delivered to relay valve service (control) port
but modulated air pressure is not being delivered, then repair or replace
service relay valve.
Symptom: Leaking exhaust port at brake, relay, quick-release valve
Suggested Action(s)
functioning correctly)
Remove delivery lines, and check for air flowing back from downstream
source. If so, identify source of air pressure. Possible internal leak in
spring brake or anti-compound check valve in spring brake relay valve.
Repair or replace component as necessary.
Repair or replace brake (treadle) valve or relay valve as necessary.
Symptom: Excessive air pressure loss on brake application
Suggested Action(s)
Perform leak-down test on air subsystems. Listen for audible source or
use soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair
as necessary.
Symptom: Vehicle pulling upon brake application
Suggested Action(s)
from relay or quick-release valve on one side
If the foundation brakes have been ruled out as the source of the
problem, then look for air restriction on delivery line to brake chamber on
side opposite direction of pull (most likely on steer axle).
Symptom: Brakes not releasing or releasing slowly after service application
Suggested Action(s)
Make brake application and release. Remove service control line. If relay
or QR valve does not release, then replace valve.
like “check valve”
Make brake application. Remove service control line. If relay or QR valve
does release, then check service line to see if there is a backward
restriction. Replace line as needed.
Make brake application and release. Remove service control line. If relay
or QR valve does release, then check service line to see if there is a
restriction. If there is no restriction, then replace brake (treadle) valve.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 13
5. Parking brake and emergency brake system
5.1 Overview
WARNING: Parking brakes are not the emergency brakes.
All buses must be equipped with an emergency brake system and parking brakes (Figure 4). The parking
brake’s function is to hold the bus stationary on a specified grade at GVWR. The parking brakes are held on
by mechanical force to ensure that they remain activated while the bus is stationary, even if air pressure leaks
away. The emergency system’s function is to provide braking force at the primary brakes, to assist the intact
secondary brake system in the event of a primary brake system failure.
The parking brake system uses a spring brake chamber that contains a powerful spring that mechanically
applies the foundation brakes. To allow normal vehicle operation, the spring force is held back by air pressure
(referred to as “hold-off air”), which releases the foundation brake. A parking brake control valve in the
operator’s compartment allows the operator to either apply this air to release the springs and drive the vehicle
or to exhaust this air to apply the springs to hold the vehicle stationary. Park control valves are typically
pressure-sensitive, normally closed, on/off valves that automatically move to the exhaust position (applying
the parking brake) when supply pressure falls below the required minimum for the particular valve installed.
A leak in the parking brake system, which causes both primary and secondary service air to be lost, will
exhaust hold-off air and cause the springs to apply, and slow a moving vehicle to a parked position or keep a
parked vehicle stationary.
The emergency brake system uses the spring brake chambers and parking brake air distribution components to
provide modulated braking on the rear axle(s) in the event of rear (primary) air reservoir pressure loss. When
primary pressure is lost, the application of the intact front (secondary) brake system causes the hold-off air to
be partially released (in proportion to the amount of front brake application, via treadle application). The
result is controlled partial application (and release) of the primary brakes via the spring brake chambers. This
provides increased stopping performance when a rear (primary) system failure occurs.
Parking brake and emergency brake systems are typically composed of six components:
air reservoirs
spring brake control valve
double check valve
dash control valve (park push-pull valve)
spring brake chambers
foundation brake components
NOTE: The combined forces of the park springs and the service air pressure may damage the
foundation brake components. Most vehicles are equipped with a feature called anti-compounding that
prevents the application of both the spring and service brake forces.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 14
FIGURE 4
Parking and Emergency Brake System Components
5.2 Troubleshooting
TABLE 3
Parking and Emergency Brake System Troubleshooting Guide
Symptom: Parking brakes don’t apply
Possible Cause(s)
Suggested Action(s)
Malfunctioning dash control valve
Confirm that the correct parking brake valve is installed. Check that the
exhaust port isn’t plugged. Check for air pressure at the inlet port. If air
pressure is not adequate, then check for restrictions upstream. If
pressure is present, then push the valve down and check for pressure in
the outlet. If not, repair or replace the valve. If pressure is present, then
check downstream.
Brake hoses damaged
Check for kinks/pinches or restrictions. Replace any damaged brake
hoses.
Malfunctioning relay valve
Remove all signal lines to the valve. If the brakes apply, then look
upstream of the relay valve. If the brakes fail to apply, then carefully
remove a spring brake hose. If the brakes release, then replace the relay
valve.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 15
TABLE 3
Parking and Emergency Brake System Troubleshooting Guide
Foundation brake problem
Diagnose and repair the problem. Reference APTA BTS-SS-RP-004-07,
“Transit Bus Front and Rear Axle S-Cam Brake Reline,” APTA BTS-BC-
RP-006-17, “Transit Bus Air Disc Brake Operation and Wheels-On
Inspection,” and APTA BTS-BC-RP-007-17, “Transit Bus Air Disc Brake
Operation and Wheels-Off Inspection.”
Inversion or spring brake control valve
problem
Make sure the valve is plumbed correctly. Refer to the OEM on the
functionality and testing of the valve.
Symptom: Parking brakes don’t release
Possible Cause(s)
Suggested Action(s)
Insufficient pressure in spring brake
chambers
Confirm adequate supply pressure. Check for restrictions in signal and
spring brake delivery system.
Chamber air leak
Check for a failed diaphragm or seal by listening for an external leak with
the spring brakes released. Check for an internal seal leak by removing
the service brake hoses, and release the spring brake. Repair or replace
the chamber if a leak is detected during either test. WARNING: Do not
disassemble spring brake chamber.
Foundation brake problem
Diagnose and repair the problem. Reference APTA BTS-SS-RP-004-07,
“Transit Bus Front and Rear Axle S-Cam Brake Reline,” APTA BTS-BC-
RP-006-17, “Transit Bus Air Disc Brake Operation and Wheels-On
Inspection,” and APTA BTS-BC-RP-007-17, “Transit Bus Air Disc Brake
Operation and Wheels-Off Inspection.”
Inversion or spring brake control valve
problem
Make sure the valve is plumbed correctly. Refer to the OEM on the
functionality and testing of the valve.
Debris in spring brake chamber
Ensure that the caging bolt plug is in place.
Malfunctioning dash control valve
Confirm that the correct parking brake valve is installed. Replace with
correct parking brake valve.
Symptom: Dash control valve doesn’t latch
Possible Cause(s)
Suggested Action(s)
Low supply air pressure at control valve
Diagnose low air pressure on inlet port.
Malfunctioning dash control valve
Confirm that the correct parking brake valve is installed. Replace with
correct parking brake valve.
Symptom: Parking brakes don’t hold
Possible Cause(s)
Suggested Action(s)
Foundation brake problem or improper
adjustment
Check for proper push rod travel, broken or damaged foundation parts,
glazed lining, lining thickness, etc. Repair as necessary. Reference
APTA BTS-SS-RP-004-07, “Transit Bus Front and Rear Axle S-Cam
Brake Reline,” APTA BTS-BC-RP-006-17, “Transit Bus Air Disc Brake
Operation and Wheels-On Inspection,” and APTA BTS-BC-RP-007-17,
“Transit Bus Air Disc Brake Operation and Wheels-Off Inspection.”
Broken or weak parking brake spring(s)
If foundation brakes are satisfactory, replace the chamber.
Incorrect brake adjuster angle/ incorrect push
rod length.
Maximum brake efficiency is achieved with brake adjuster at 90 deg.,
±10 deg. Verify that foundation brake is set up according to the
manufacturer’s specifications.
Caging bolt engaged
Disengage the caging bolt.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 16
TABLE 3
Parking and Emergency Brake System Troubleshooting Guide
Symptom: Parking brakes are slow to apply
Possible Cause(s)
Suggested Action(s)
Foundation brake problem or improper
adjustment
Check for proper push rod travel, return springs, binding in camshaft or
anchor pins, broken or damaged foundation parts, etc. Repair as
necessary. Reference APTA BTS-SS-RP-004-07, “Transit Bus Front and
Rear Axle S-Cam Brake Reline,” APTA BTS-BC-RP-006-17, “Transit
Bus Air Disc Brake Operation and Wheels-On Inspection,” and APTA
BTS-BC-RP-007-17, “Transit Bus Air Disc Brake Operation and Wheels-
Off Inspection.”
Restriction in the air system
Check hoses for kinks/pinches or restrictions. Replace any damaged
brake hoses. Check valves for contamination and proper operation.
Brake chamber malfunction
Repair or replace the brake chamber as necessary.
Symptom: Parking brakes are slow to release
Possible Cause(s)
Suggested Action(s)
Low air system pressure
Diagnose air system pressure. Refer to Section 3.1.
Foundation brake problem or improper
adjustment
Check for proper push rod travel, return springs, binding in camshaft or
anchor pins, broken or damaged foundation parts, etc. Repair as
necessary. Reference APTA BTS-SS-RP-004-07, “Transit Bus Front and
Rear Axle S-Cam Brake Reline,” APTA BTS-BC-RP-006-17, “Transit
Bus Air Disc Brake Operation and Wheels-On Inspection,” and APTA
BTS-BC-RP-007-17, “Transit Bus Air Disc Brake Operation and Wheels-
Off Inspection.”
Restriction in the air system
Check hoses for kinks/pinches or restrictions. Replace any damaged
brake hoses. Check valves for contamination and proper operation.
Brake chamber malfunction
Repair or replace the brake chamber as necessary.
Leak in the air system
Perform leak-down test on air subsystems. Listen for audible source or
use soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair
as necessary.
Contaminated valve
Replace or repair valve as necessary. Check air compressor/dryer
system. Drain moisture from air tanks. Replace contaminated
component
Symptom: Self-application
Possible Cause(s)
Suggested Action(s)
Air leak in delivery circuit
Perform leak-down test on air subsystems. Listen for audible source or
use soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair
as necessary.
Low system pressure
Diagnose air system pressure. Refer to Section 3.1.
Brake chamber malfunction
Repair or replace the brake chamber as necessary.
Malfunctioning dash control valve
Confirm that the correct parking brake valve is installed. Replace with
correct parking brake valve.
Inversion or spring brake control valve
problem
Make sure the valve is plumbed correctly. Refer to the OEM on the
functionality and testing of the valve.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 17
TABLE 3
Parking and Emergency Brake System Troubleshooting Guide
Contaminated valve
Replace or repair valve as necessary. Check air compressor/dryer
system. Drain moisture from air tanks. Replace contaminated
component.
Symptom: Dash indicator problem
Possible Cause(s)
Suggested Action(s)
Faulty switch
Replace the switch and or bulb.
6. Accessory system
6.1 Overview
Most buses are equipped with an accessory air tank. Air is supplied to the accessory tank by the supply
system. The pressure protection valve allows pressure to build in the brake reservoirs to the set opening
pressure before opening and allowing the accessory tank to pressurize and equalize with the other brake
reservoirs. If the accessory tank fails or all air is lost from the accessory tank, then the pressure protection
valve closes at the set closing pressure (typically 10 to 15 psi lower than the opening pressure), ensuring that
pressure is retained in the braking system reservoirs. The tank also is equipped with a drain valve.
Prior to diagnosing any accessory, it is important to confirm adequate air supply.
The following components are typically supplied by the accessory tank:
leveling valves
kneeling valves
wiper motor
driver’s seat
spinner filter
entrance door motor
exit door motor
6.2 Troubleshooting
TABLE 4
Accessory System Troubleshooting Guide
Symptom: Accessories operating slowly or not operating
Possible Cause(s)
Suggested Action(s)
Insufficient air supply
Diagnose air system pressure. Refer to Section 3.1.
Air system leak
Perform leak-down test on air subsystems. Listen for audible source or use
soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair as
necessary.
Pressure protection valve not opening
Check accessory tank pressure. If pressure is low, then replace valve.
Restricted lines
Check for kinked hose or blocked tubing, and leaking or cracked air lines.
Repair and replace.
Moisture in air tank
Drain moisture from air tanks. Check air compressor/dryer system. Replace
contaminated component(s).
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 18
TABLE 4
Accessory System Troubleshooting Guide
Symptom: Suspension not leveling
Possible Cause(s)
Suggested Action(s)
Insufficient air supply
Diagnose air system pressure. Refer to Section 3.1.
Air system leaks
Perform leak-down test on air subsystems. Listen for audible source or use
soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair as
necessary.
Pressure protection valve not opening
Check accessory tank pressure. If pressure is low, then replace valve.
Restricted lines
Check for kinked hose or blocked tubing. Repair or replace.
Leveling valve control arms
Inspect linkage for looseness, flipping or other damage. Repair or replace as
necessary.
Moisture in air tank
Drain moisture from air tanks. Check air compressor/dryer system. Replace
contaminated component(s).
Air spring
Check air spring for leaks. Replace as necessary. Check for mismatched
parts.
Symptom: Suspension not kneeling (lowering)
Possible Cause(s)
Suggested Action(s)
Electrical malfunction
Refer to the electrical section in the OEM manual.
Kneel valve out of adjustment
Refer to the adjustment procedures in the OEM manual for the kneeling
system.
Symptom: Suspension not recovering (raising)
Possible Cause(s)
Suggested Action(s)
Kneel valve malfunction
Refer to the adjustment procedures in the OEM manual for the kneeling
system.
Level valve control arm(s) out of
adjustment
Inspect for damaged, bent, broken or flipped arm. Repair or replace as
necessary.
Electrical malfunction
Refer to the electrical section in the OEM manual.
Malfunctioning quick-fill valve
Check valve operation. Repair or replace as necessary.
Air supply
Diagnose air system pressure. Refer to Section 3.1.
Symptom: Pneumatic door operation, entrance door, exit door (not opening or closing)
Possible Cause(s)
Suggested Action(s)
Air supply
Diagnose air system pressure. Refer to Section 3.1.
Air system leaks
Perform leak-down test on air subsystems. Listen for audible source or use
soapy water to isolate the leaks (chamber, valve, hose, etc.). Repair as
necessary.
Restricted lines
Check for kinked hose or blocked tubing. Repair and replace as necessary.
Pressure protection valve not opening
Check accessory tank pressure. If pressure is low, then replace valve.
Door out-of-mechanical adjustment
Check mechanical linkage for binding and sticking. Refer to the adjustment
procedures in the OEM manual for door speed adjustments.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 19
TABLE 4
Accessory System Troubleshooting Guide
Rotary valve (air shutoff valve)
Check operator door control valve.
Contaminated valve
Check air compressor/dryer system. Drain moisture from air tanks Replace
contaminated component.
Symptom: Driver’s seat not operating
Possible Cause(s)
Suggested Action(s)
Supply line
Check supply air line to driver’s seat. Check for kinked hose, blocked tubing,
leaking or cracked air lines. Repair or replace as necessary.
Insufficient air supply
Diagnose air system pressure. Refer to Section 3.1.
Control valves (air shutoff valve)
Close or replace defective drain valves.
Pressure protection valve
Check accessory tank pressure. If pressure is low, then replace valve.
Symptom: Malfunctioning wipers
Possible Cause(s)
Suggested Action(s)
Insufficient air supply
Diagnose air system pressure. Refer to Section 3.1.
Mechanical linkage
Check mechanical linkage for wear, damage, binding or sticking. Lubricate,
replace or rebuild as possible.
Control valves
Check pressure at supply and delivery port at the valve. If supply pressure is
low, then check for restrictions between pressure protection valve and
control pressure. Repair or replace as necessary. If delivery pressure is low,
then replace valve. If delivery pressure is adequate, then check wiper motor.
Wiper motor
Refer to OEM manual for proper test procedure.
APTA BTS-BC-RP-010-20
Troubleshooting Transit Bus Air Systems
© 2020 American Public Transportation Association 20
Related APTA standards
APTA BTS-SS-RP-004-07, “Transit Bus Front and Rear Axle S-Cam Brake Reline”
APTA BTS-BC-RP-006-17, “Transit Bus Air Disc Brake Operation and Wheels-On Inspection”
APTA BTS-BC-RP-007-17, “Transit Bus Air Disc Brake Operation and Wheels-Off Inspection”
APTA BTC-BC-RP-008-19, “Transit Bus Brake Valve Treadle Assembly Maintenance”
Abbreviations and acronyms
AC alternating current
CCOHS Canadian Centre for Occupational Health and Safety
GVWR gross vehicle weight rating
NATSA North American Transportation Services Association
OEM original equipment manufacturer
OSHA Occupational Safety and Health Administration
PLC programmable logic controller
psi pounds per square inch
QR quick release
RPM revolutions per minute
RTV room temperature vulcanizing
V volt
Document history
Document
Version
Working Group
Vote
Public Comment/
Technical
Oversight
CEO Approval
Policy &
Planning
Approval
Publish Date
First published July 3, 2019 Sept. 13, 2019 Nov. 15, 2019 Jan. 31, 2020 Feb. 4, 2020