Air configuration from tractor to trailer - baffling!

Howdy folks, Im building a truck that was converted to a wrecker about 30 years ago (84 Marmon) and now converting it back to a tractor. Today I got my trailer air lines installed and "mostly" working right. The thing is that the trailer air will not supply air unless the tractor park brake is released. With the tractor brake on, it makes no difference at all if the trailer brake is in or out. Is this some kinda old school way of doing things, or what am I missing?

 

Its only got the 2 valves...yellow n red.

 

Im assuming they re-rigged a bunch of stuff when they put the wrecker body on...so yea Im thinking Im gonna have to just start following lines

 

Hey fella's, so today i fired it up again to check on a couple other things and i discovered that when i step on the brake pedal with the tractor park brake released, i have a good amount of air coming from the hole on top of this valve. Its supposed to be open since its not threaded. What's going on here?

 

Trying to add the pic