Peterbilt Model 320 Operator's Manual after 8-07 - Peterbilt Motors ...

PART 7: STARTING & OPERATING THE VEHICLE
• To release the full combination of brakes, push in
BOTH yellow and red knobs.
Trailer Air Supply Valve
The red octagon knob controls the air supply to the trailer.
To supply air to the trailer system and release the trailer
parking brakes:
• Allow the tractor air system pressure to build up to
operating level.
• Pushtheredknobin.
• Holdtheredknobinbyhanduntilthetrailerairpressure
builds to a pre-set level, about 45 psi. At this point
it will remain in.
The yellow knob will remain out; the tractor will remain
parked. If you ever have a failure or disconnect of the air
supply hose to the trailer, the trailer parking brakes will set.
The red knob will automatically pop out and seal off the
tractor air reservoirs to protect the tractor air system pressure.
When operating the tractor “bobtail”: The red knob
must remain out to protect the tractor air system pressure
Manually Releasing Spring Brakes
Manually Releasing Spring Brakes
There may be an occasion when you need to tow the vehicle
but the air system does not produce enough operating
pressure to release the parking brakes. For these situations,
the spring brakes may be manually released to allow
the vehicle to be towed. This sections details how to release
a drum or an air disc brake.
WARNING!
• Do not operate a vehicle when the spring
brakes have been released manually. Driving
a vehicle that has had its spring brakes
manually released is extremely dangerous.
You would probably have no brakes at all.
You could have a serious or fatal accident.
• Releasing the spring brakes on an unsecured
vehicle could lead to an accident. The truck
could roll, causing severe injury. Always secure
the truck with wheel chocks, chains, or
other safe means to prevent rolling before
manually releasing the spring brakes.
R(08/07) Y53-6015 – 95 –