Air build up problem!

You most likely have an unloader valve hanging and passing some air when the air compressor is cold and when compressor warms up it functions normal. Install an unloader kit and see if that cures the problem.

 

If it's not building up at his to be leaking someplace. Can you locate the leak? f the governor was shot I can't figure out why it would only build up at higway speed.

 

A leaking unloader will leak into the air compressor intake side so remove the intake hose and listen closely.

 

The air compressor won't pump air with one of the unloader valves hung open.

 

On the highway the engine is loaded so it builds boost if the comp gets its air from the intake manifold it is getting pressure to the unloaders. That may affect a stuck unloader [ temp press ] just saying. But there is a check valve between the tank and the comp so it shouldnt leak down through the comp!!

 

Although I have installed many check valves on the wet tank, I don't recall seeing them factory installed.
The air that would be leaking back through the compressor would be coming from the secondary tank and back through the signal line to the air governor then onto the unloader valves.

 

Got ya i belive all the trucks i have had or at least the ones i had problems in that line or tank had them. thought it was standard. So if thats the case theres the leak!!