Has anyone ever worked on one or rebuilt one? This one seems to be on the frame under the tilt cab. Or is it just a warning of loss of pressure on one side of the brake system switch that is there and is leaking? Help!!!

 

You need a factory manual that covers the functions of the valve. They have a switch that monitors the position of an internal plunger. The plunger moves to one side if a brake line ruptures sealing that side off. There may also be a pressure compensation valve inside.

Check with the online store here to see if the Factory service manuals they have on CD cover the HD trucks. My paper manuals cover the heavy trucks but I don't have your year.

 

I have never seen any F Series Ford Truck with tilt Cab. Are you sure you have an F600?

Also what you seem to be talking about is Air Brakes not Vacuum over Hydraulic, or Hydra Vac Assist.

Air brakes have a B1 splitter valve facilitatating different ways to charge system with air @PSI to activate the air brakes. . . . .
FWIW, foot valve, trolley valve & Park Valve all operate the brake system, but do it through one set of lines to chambers, not 3sets.

I am totally confused

 

It could be a C-600 COE style.