Ford F-650 2015 Owner's Manual page 148

You can push in the parking brake (B – yellow knob) and supply air to
the tractor spring brakes, releasing them.
Normal Driving Position
Push in both knobs to supply air to both trailer and tractor spring
brakes, releasing all brakes.
2015 F-650/750 (f67)
Owners Guide gf, 2nd Printing, April 2014
USA (fus)
Brakes
B
A
B
A
147