Mack CHM 2010, Operator'S Handbook Manual

Page: 125 / 232

OPERATION 117
BRAKE OPERATION
Air Brake System
This chassis features a dual braking system which has two complete air circuits: a primary
circuit for rear brakes and a secondary circuit for front brakes. Each circuit receives air from
separate reservoirs. Although there are two air circuits, they operate as one brake system
through the dual-circuit treadle valve. This provides the driver with easy, graduated control
when applying and releasing the brakes.
The air pressure in the two circuits is monitored by gauges on the instrument panel. When
air pressure drops below 517 ± 34 kPa (75 ± 5 psi) in either system at any time other than
vehicle startup, pull to the side of the road and determine the problem. If air pressure
continues to drop below 276 ± 34 kPa (40 ± 5 psi) in BOTH systems, spring brakes will
automatically apply. The Low Air Pressure Warning indicator and buzzer will be activated
if low air pressure occurs in either circuit.
In tractor applications, the Trailer Supply Valve (red octagonal knob) will immediately pop
out in the event of a trailer breakaway or sudden trailer air line failure, which will apply
the trailer spring brakes. In the event of a slow leak in the trailer air system, the trailer
supply valve will pop out when system Pressure reaches 483 kPa (70 psi). This protects the
tractor air system from further pressure loss.
If the Trailer Supply Valve is held in, in an attempt to override application of the trailer spring
brakes, the Park Brake Valve (yellow diamond knob) will automatically pop out and apply
the parking brakes when system pressure drops to approximately 138–207 kPa (20–30 psi).
The air brake system consists of three main elements:
•
The compressor, governor and reservoirs supply and store the air pressure.
•
The brake application valve controls the brake application pressures.
•
The brake chambers control the brake mechanism.
Air Brake Operation
CAUTION
Avoid sudden stops. Constant, sudden
stops may negatively affect the
performance of braking and driving parts.
When slowing for a stop, leave the clutch engaged for as long as possible to use the braking
effect of the engine. When forward speed has dropped to a little above idling speed, push
clutch pedal in and brake to a complete stop.