Ford F-650 2001 Owner's Manual page 119

Driving
Do not use the trailer brake manual control lever for parking.
Trailer air supply control (MV-3) (Tractor package)
The MV-3 control module is a two-button, push-pull control valve housed
in a single body which includes a dual circuit supply valve and a check
valve.
Operation
During initial charge, with the system completely discharged, both
buttons are out.
When system pressure reaches
448 kPa (65 psi), the red control
(trailer supply) may be pushed in
TO
and should stay in, charging the
trailer system and releasing the
TRAILER
trailer brakes.
AIR SUPPLY
NOT FOR PARKING
TO
The yellow control (parking brake)
may now be pushed in, which will
TO
supply air to the tractor spring
brakes, releasing them.
PARKING
BRAKE
TO
Normal operating position
With both controls pushed in, air is supplied to the trailer and to the
tractor spring brakes; all brakes are released.
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