Ford F-650 2008 User Manual page 108

Driving
The parking brake (2–yellow knob) can now be pushed in and supply air
to the tractor spring brakes, releasing them.
Normal driving position
With both knobs pushed in, air is then supplied to both trailer and
tractor spring brakes, and all brakes are released.
108
2008 F-650/750 (f67)
Supplement