Ford F-650 2006 Manual page 113

To apply the trailer brakes using the hand control, move the lever
downward. The further the lever is pushed downward, the greater the air
pressure is applied to the brakes. The lever will remain in place until
manually moved.
To release the trailer brakes, move the lever upward completely.
TRAILER AIR SUPPLY AND PARKING BRAKE MODULAR
CONTROLS (IF EQUIPPED)
The trailer air supply valve delivers air to the trailer supply and will
automatically pop out, shutting off the trailer supply if pressure is
decreased to approximately 35 psi (249 kPa).
The parking brake controls the spring brakes on the tractor. When the
knob is pulled out it causes the trailer supply valve to pop out, applying
both the tractor and trailer parking brakes. The trailer brakes may be
independently released by pushing only the trailer air supply valve in.
2006 F-650/750 (f67)
Supplement
USA (fus)
TO
TRAILER
AIR SUPPLY
NOT FOR PARKING
TO
Driving
113