Medium Duty Operator's Manual - Peterbilt Motors Company

STARTING AND OPERATING THE VEHICLE
WARNING! If the air pressure falls below 60
psi (414 kPa) the spring brakes may stop the
vehicle abruptly, which could result in an
accident and/or injuries. Observe the red
warning lamps on the gauges. If one comes
on, do not continue to drive the vehicle until
it has been properly repaired or serviced.
WARNING! Do not use the trailer hand brake
or service brakes to park and hold an unattended
vehicle—use the parking brakes.
Because service brakes work with air pressure,
these brakes could slowly release. Your
vehicle could roll, causing a serious accident.
Someone could be hurt or killed. Never
rely on the service brakes to hold a parked
vehicle.
Tractor and Trailer Air Supply Valve
Initial Charge
The red octagon knob controls the air supply to the trailer.
With the system completely discharged, both the Red
(trailer air supply) and the Yellow (parking brake) knobs
are OUT; thus, tractor and the trailer parking (spring)
brakes are applied.
USING THE BRAKE SYSTEM
To supply air to the trailer system and release the trailer
parking brakes:
1. Allow the tractor air system pressure to build up to
operating level.
2. When system pressure reaches 50 psi (345 kPa) the
Red knob may be pushed IN.
3. Hold the Red knob IN by hand until the trailer air pressure
builds to a pre-set level, about 45 psi. At this
point it will remain in, charging the trailer system and
releasing only the trailer brakes.
4. The Yellow knob will remain OUT (tractor brakes ON).
Normal Run Position
• The Yellow knob (system park) may now be pushed
IN, which will supply air to the tractor spring brakes,
releasing them.
• With both knobs pushed IN, air is now being supplied
to both the trailer and the tractor spring brakes; all
brakes are released.
Medium Duty (R05/09) Y53-6008A – 105 –