Using the Brake System Operating Instructions
– 76 – Y53-6008
Medium Duty (R10/06)
WARNING! If the air pressure falls below 60 psi
(414 kPa) the spring brakes may stop the vehi-
cle abruptly, which could result in an accident
and/or injuries. Observe the red warning lamps
on the gauges. If one comes on, do not con-
tinue to drive the vehicle until it has been prop-
erly 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 ser-
vice 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.
To supply air to the trailer system and release the trailer park-
ing brakes:
1. Allow the tractor air system pressure to build up to oper-
ating 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 pres-
sure 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, releas-
ing them.
• With both knobs pushed IN, air is now being supplied to
both the trailer and the tractor spring brakes; all brakes
are released.
