Chapter 31
Air Brake Servicing
Objectives (1 of 3)
• Understand the safety requirements of
working on an air brake system.
• Perform basic maintenance on an air brake
system.
• Diagnose common compressor problems.
• Describe the procedure required to service
an air dryer.
• Performance test an air dryer.
Objectives (2 of 3)
• Check out the service brakes on a truck.
• Test the emergency and parking brake
systems.
• Verify the operation of the trailer brakes.
• Understand the OOS criteria used by safety
inspection officers.
• Diagnose some brake valve failures.
• Describe the procedure required to overhaul
foundation brakes.
Objectives (3 of 3)
• Determine brake free-stroke and identify
when an adjustment is required.
• Outline some common service procedures
used on air disc brake systems.
Shop Talk
• When replacing brake hose and fittings, be
sure to use the same sizes, or the movement
of air through the system will be altered.
• One air brake OEM has determined that
replacing a straight pipe nipple with a 90degree elbow is equivalent to adding 7 feet of
brake hose into the circuit. Adding a 45degree elbow is equivalent to adding 3 feet of
hose into the circuit.
Brake Torque Balance
Caution
• All air tanks that are FMVSS 121-compliant
must have a means of mechanically draining
them.
• Most automatic drain valves are equipped
with a mechanical dump valve, but it can be
difficult to locate. One design uses a
Schrader-type plunger that is difficult to
access.
Air Tanks
Air Brake Test Equipment
Double Gladhand (1 of 2)
Double Gladhand (2 of 2)
Governor
Shop Talk
• Maximum allowable leakage/drop-off rates
are often defined by local jurisdictions (state
and provincial governments).
• These regulations may differ from the test
values used here. Contact your local
transportation enforcement office to identify
the specifications used in your area.
Dash Control Valve Module
Push-Pull Valve
Caution
• All compressors pass minute quantities of oil
into the air circuit.
• Avoid replacing a compressor that is passing
trace oil because the warranty will usually be
denied by the OEM.
• The warranty also will be denied if the drainback ports are plugged with silicone.
Air Compressor
Compressor Problems
• See Table 31-1 on page 1010 of the
textbook.
Air Drier
Typical Foot Valve
Shop Talk
• Carefully label and code every line as it is
removed from the foot valve assembly.
– Failure to do this can considerably lengthen
the time required to install the replacement
valve.
Typical Relay Valve
Quick Release Valve
Warning
• Always cage the compression spring with the
release tool.
• Never rely on air pressure to keep the spring
compressed.
Air Chamber Release Tool
Air Chamber Service (1 of 3)
Air Chamber Service (2 of 3)
Air Chamber Service (3 of 3)
Caution
• Never open a disarmament chamber until the
main spring has been observed through the
inspection windows to be separated from the
housing and is not under tension.
Caution
• The loaded main spring in a spring brake
assembly contains a potentially lethal force.
• Manual caging devices must be used
whenever the spring chamber assembly is
removed and replaced from the foundation
brake mounting plate.
• Most current spring brake units are sealed
with a band clamp that has to be destroyed to
separate the spring chamber in order to
prevent the unit from being disassembled.
Pushrod Length (1 of 5)
Pushrod Length (2 of 5)
Pushrod Length (3 of 5)
Pushrod Length (4 of 5)
Pushrod Length (5 of 5)
Slack Adjuster
Caution
• The clevis must be installed in the correct
position on the pushrod, or the slack adjuster
will not adjust the brake correctly.
Shop Talk
• The adjusting pawl assembly can be on
either side of the housing or on the front of
the housing.
• Make sure that the pawl assembly can be
removed after the slack adjuster is installed.
• The pawl assembly must sometimes be
removed when the slack adjuster is serviced.
Caution
• If the pawl is not removed, the teeth will be
damaged when the manual adjusting nut is
turned.
Air Valve Troubleshooting Guide
• See Table 31-2 on page 1029 of the
textbook.
Shoe Replacement (1 of 5)
Shoe Replacement (2 of 5)
Shoe Replacement (3 of 5)
Shoe Replacement (4 of 5)
Shoe Replacement (5 of 5)
Caution
• On 16.5-inch brake shoe foundation
assemblies using 1-inch anchor pins, the
spider anchor pin bore must not exceed
1.009 inches (25.63 mm).
Shop Talk
• Used drums are almost never machined and
returned to service today.
– This is because of the low cost of new
replacement drums when compared to the
labor required to turn a drum.
– Additionally, machining used brake drums is
technically not a good practice, as the drums
have a much shorter in-service life because of
much harder friction linings.
Caution
• Cast drums are machined in manufacture.
However, when stored, especially for long
periods, they can deform and lose their
concentricity.
– It is, therefore, a good practice to machine
brand new drums to ensure that they are
perfectly concentric with the wheel.
Install the S-cam and Shoe
Left and Right Side Orientation
Shop Talk
• Modern friction facings on brake shoes are much
harder than the asbestos base linings used on
drums a generation ago.
• They have a longer service life.
– The result is that the life of the linings can be the
same as the life of the drums in some applications.
– It is almost impossible to machine/turn down the
effects of severe heat checking on a brake drum
because the drum becomes so hardened. Cast brake
drums are cheap when compared with the labor cost
required to turn a drum. Many operations routinely
replace brake drums today when the brakes are
relined.
Shop Talk
• When an axle wheel seal fails, oil saturates
the brake friction linings, which rapidly
causes them to glaze, lowering their
coefficient of friction or aggressiveness.
– When this occurs, pressure or steam washing
removes only the surface oil, so the linings
should be replaced.
– Also replace the linings on the other end of the
axle to maintain brake torque balance over the
axle.
Shop Talk
• Take some time to ensure that automatic
slack adjusters are properly set up on
installation.
• It takes much longer to manually adjust
automatic slack adjusters than manual slack
adjusters.
Shop Talk
• Avoid lubricating the cam roller face.
• Lubricate the cam roller bearings.
– These are designed to grab at their faces and
rotate on their bearings in operation.
Shop Talk
• Out-of-adjustment brakes affect all vehicle
braking, including emergency braking.
• Never take chances with brake adjustment.
– Maladjusted brakes can have lethal
consequences, and every technician has a
stake in the vehicles on which they work.
Shop Talk
• Avoid lubricating the cam roller face.
– Lubricate the cam roller bearings.
– These are designed to grab at their faces and
rotate on their bearings in operation.
Shop Talk
• Out-of-adjustment brakes affect all vehicle
braking, including emergency braking.
• Never take chances with brake adjustment.
– Maladjusted brakes can have lethal
consequences, and every technician has a
stake in the vehicles on which they work.
Checking Free-stroke
Shop Talk
• Brake freestroke is typically specified to be
between 3/8 (0.375 inch) and 5/8 (0.625 inch)
regardless of chamber size.
• Check the OEM specifications.
Air Disc Brakes
Checking Rotor Runout
Summary (1 of 3)
• Brake system maintenance and service procedures
attempt to ensure that air-brake-equipped vehicles
meet FMVSS 121 standards when in service.
• Brakes must meet their original standards in terms
of pneumatic timing and brake torque balance.
• Pneumatic timing refers to the management of the
air in the system to ensure that the air reaches each
actuator at the correct time so that the brakes over
one wheel are not applied before or after those on
another wheel.
Summary (2 of 3)
• Brake torque balance refers to the requirement that
the same amount of mechanical force be delivered
to the wheels on each axle and that the brakes over
all the axles on a vehicle work together to bring the
vehicle to a stop.
• Disc brake assemblies are increasingly used in
truck applications despite higher costs because of
their superior brake performance. Service
procedures required on disc brake assemblies
require you to be able measure lining wear,
lubricate calipers, and measure rotor runout.
Summary (3 of 3)
• Minimum training requirements and
qualifications are necessary for employees
who re-inspect, repair, maintain, adjust, or
periodically inspect the brakes on
commercial motor vehicles (CMVs).
• Roller brake dynamometers and plate testers
are used in brake inspection testing.