Ford 2013 F-650 Owner's Manual page 158

Automatic Application
If both knobs are pushed in and the brake system air pressure is reduced
to approximately 35 psi (249 kPa), the trailer air supply (A) knob
automatically pops out applying the emergency or parking brakes on the
trailer. If the trailer air supply (A) knob is manually held in and the air
pressure is reduced to approximately 30 psi (207 kPa), a tripper piston
within the valve moves, exhausting the trailer air supply, applying the
trailer brakes. Further reduction of air pressure, while holding the trailer
air supply knob in, causes the parking brake knob to pop out at 25 psi
(172 kPa).
Actuation of Trailer Park (Emergency) or Tractor Bobtail Position
To actuate the trailer brakes only, pull out the trailer air supply knob
(A). The trailer brakes are now applied whether emergency or spring
brakes are used on the trailer.
This mode is also used when the tractor or truck with trailer is used
during bobtail operation.
2013 F-650/750 (f67)
Owners Guide gf, 2nd Printing, March 2013
USA (fus)
Brakes
B
A
B
A
157