Mack TerraPro LEU 2010, Operator'S Handbook Manual

Page: 135 / 225

124 OPERATION
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 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 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 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.