2010-02 Air Brake Systems

2010-02 Air Brake Systems
Service Tips
WIT Club News – February/March 2010
This month’s article is contributed by our
chassis-engineering group and explains the
essential function of the air brake systems in
our diesel vehicles. We hope this will be
helpful to those of you who own a diesel
coach, and interesting to those of you who
Air pressure is produced by an engine driven
air compressor. The compressor is controlled
by a governor which senses reservoir pressure
and activates pumping below approximately
100 # P.S.I. and stops pumping at 120 – 125 #
A high temperature flexible line moves air
from the compressor through a check valve
and into the air dryer. The air dryer, with
heated drain, automatically purges collected
moisture each time the compressor reaches
maximum pressure.
Air from the air dryer enters the first, small,
chamber of the primary reservoir.
chamber is sometimes referred to as a wet
tank. As the hot compressed air cools, it loses
more moisture which can be collected in this
chamber. A drain valve is located in the
bottom of this chamber to drain the moisture.
This chamber also provides pressure control
signal to the compressor governor and an
excess pressure relief valve.
Air leaving the wet tank now goes through
two separate check valves to two separate
reservoirs, the large chamber of the primary
reservoir and the large chamber of the
secondary reservoir creating a “split system”.
The primary system controls the rear
(powered) axle and the secondary system
controls the front axle. Both chambers have
drain valves.
Drain valves are either manual or automatic.
It is the operator’s responsibility to operate the
manual valves to drain moisture.
Accessory (non air brake function) air for air
suspension, air horns, and customer access
air station (quick connect) exits the secondary
reservoir through a pressure protection valve.
If the pressure in the reservoir is less than the
protection valve set point (approximately 65 #
P.S.I.), it will shut off air to the accessories and
keep it in the reservoir for the brakes.
From the two independent reservoirs, the air
goes to two independent valves operated by
the brake pedal. When the driver steps on the
pedal, the pedal movement controls the air
through the two independent valves; one to the
front axle, and one to the rear axle.
Air controlled by the brake pedal arrives at
the brake chambers (two for each axle, one for
each wheel). Mechanical movement from the
components in the end of the axle move either
the brake shoes outboard to contact the brake
drums or disc brake pads inboard to contact
the rotor. Normal service brake functions on
both axles use air pressure controlled by the
brake pedal to apply the brakes.
The rear brake chambers are a double
diaphragm with two chambers that serve two
different functions. One is normal service
brake operation as described above, the other
is park brake or low air spring brake apply.
When the brake system has normal pressure
and the park brake is NOT applied, system air
pressure present in each spring brake
compresses a heavy spring. The hiss during
park brake application is air being released
from the chambers, allowing the strong springs
to expand within the chamber and the brake
shoes to contact the brake drums. In the
event system air pressure is inadvertently lost,
these springs will also apply the rear brakes. If
the compressor will not rebuild sufficient
system air pressure, corrective repair must be
done to release the brakes before the vehicle
can be moved.
During the 1999 chassis model year, ABS
(Anti-Lock Brake System) became standard.
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