Replace Air Regulator???

Hello guys. I have a 1993 Peterbilt 379 project truck and it's been setting for a couple years but finally getting around to working on it. The brakes have always made a steady "air horn toot" sound as soon as you press down on them (not the usual air sound when letting off). When I started it the other day, I noticed the air compressor regulator/governor on the side of the engine was spewing air. The truck has always had trouble building air. I plan to pick up a new regulator in a couple days. Could this be why the brakes have always made this sound?
If not, what should I check? A valve???

 

I've got 2 other trucks this age (another '93 and a '92), neither make this sound with the brakes

 

The sound is definitely coming from behind the dash at the park brake valve area.

 

Yes