Ford F-650 2011 Supplement Manual page 127

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.
127
2011 F-650/750 (f67)
Supplement, 2nd Printing
USA (fus)