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Brakes 229
Parking Brake/Trailer Supply Valves
Trucks/Tractors may have two air control
valves on the instrument panel:
•
Trailer Supply (red octagonal knob)
•
System Park (yellow diamond knob)
The two air control valves are intercon-
nected and interacting. The System Park
valve supplies and controls the Trailer Air
Supply valve.
The System Park valve knob (yellow)
should be pushed in first, after sufficient air
pressure is built up (apply foot brake to pre-
vent vehicle from rolling). The Trailer Air
Supply valve knob may then be pushed in.
The Trailer Air Supply valve knob and Sys-
tem Park valve knob will automatically pop
out if the system pressure (both front and
rear circuits) drops to 25 to 35 psi (170 to
240 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 to-
gether with the other permits the operator
to select the desired functions described be-
low:
Red Valve
(Trailer Air
Supply)
Yellow
Valve (Sys-
tem Park)
Function
(Mode)
Out Out System
Park
In Out Trailer
Charge
In In Normal
Running
Out In Bobtail
W5000924
W5001138