Ford 750 2009 Owner's Manual page 117

Driving
System park
With both knobs pushed in (normal driving position), the parking brakes
for both the tractor and trailer can be applied by pulling the parking
brake knob (2) out, exhausting air from the tractor spring brakes,
simultaneously causing the trailer air supply valve to pop out, applying
the trailer brakes.
Trailer charge
If both knobs are out, and you want to recharge the trailer while leaving
the tractor spring brakes applied, the trailer air supply (1) can be
pushed in to recharge the trailer air supply line. This mode may also be
used to park a combination vehicle with tractor spring brakes.
117
2009 F-650/750 (f67)
Supplement
USA (fus)