Operation
Trailer Air Supply and Parking Brake Modular Controls
• TRAILER AIR SUPPLY (red octagonal knob)
• PARKING BRAKE (yellow diamond knob)
The PARKING BRAKE valve knob (yellow) should be pushed in
first, after sufficient air pressure is built up (apply foot brake to
prevent vehicle from rolling). The TRAILER AIR SUPPLY valve
knob may then be pushed in.
The TRAILER AIR SUPPLY valve knob (red) and PARKING
BRAKE valve knob will automatically pop out if the system
pressure (both front and rear circuits) drops to 20 to 40 psi
(138 to 276 kPa). The tractor protection valve will then close,
the tractor spring brakes will apply, and the trailer emergency
system will be activated.
On vehicles equipped with the standard two-valve system,
the operation of one valve together with the other permits the
operator to select the desired functions described below:
Red Valve
(Trailer Air Supply)
Yellow Valve
(Parking Brake)
Function (Mode)
Out Out System Park
In Out Trailer Charge
In In Normal Running
Out In Bobtail/Sliding
Fifth Wheel
Adjustment/Sliding
Trailer Tandems
Adjustment
The PARKING BRAKE valve (yellow knob) controls the spring
brakes on the tractor and when pulled out simultaneously causes
the trailer supply valve to pop out, thus applying both tractor and
trailer parking brakes. The trailer brakes may be independently
released by pushing only the TRAILER AIR SUPPLY valve (red
knob) in.
The TRAILER AIR SUPPLY valve (red knob) delivers air to the
trailer supply and will automatically pop out, shutting off the trailer
supply if pressure is decreased to approximately 35 psi (241
kPa). For exact air pressure set points, refer to the Service
Manual.
152 3879634R1
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