Ford 2013 F-650 Owner's Manual page 156

The parking brake (B – 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 supplied to both trailer and tractor
spring brakes and all brakes are released.
2013 F-650/750 (f67)
Owners Guide gf, 2nd Printing, March 2013
USA (fus)
Brakes
B
A
B
A
155