2002 Peterbilt 379 where is my air leak?

Ok. After monkeying with this thing for 2 days, here's where Im at.

Installed new unloader valves. Truck wouldn't build any air pressure. Pull supply air, air compressor puts out air. Pull dryer filter. Pretty nasty underneath, so I clean it out. Remove and install new purge valve. Truck begins to build air, but now the purge valve starts leaking at about 95psi until its so bag the compressor wont built past 110psi. I adjusted the gov down until it fully purged at 110, then it almost immediately starts leaking again.

Going over the service data diagram, I dont see anything that would cause this besides maybe the delivery check valve?

Everything else in the system is new. Do I bite the bullet and replace the dryer?

Edit:

Here's my working theory. The unloader valves were worn and weak, covering up a bad possibly check valve in dryer which once the gov kicked in and purged the system, the weak unloader valves would stay...open? and allow the weak delivery port check valve to allow air to escape back into the compressor and vent back through the intake port (mine has a line going into the head) until the pressure dropped enough that the unloader valves would close and thus stop leaking air enough that it wasnt audible any more.

 

Definitely gonna try that. I did have the signal line off the governor yesterday while building air pressure at one point, but I didnt have the port plugged, so I'll do that.

I did find a World Products dryer through my semi local Carquest that does HD truck parts as well yesterday and they claim to have it in stock, so I'm gonna check it out and see if its correct. Otherwise Fleetpride in San Antonio has them in stock.

 

So update on this. I finally got a new air dryer installed. My working theory was right, the air dryer was leaking from the delivery port check valve back through the air compressor to the weak unloader valves. New unloader valves really allowed the bad aid dryer to show itself.

So now the truck is about as air leak free as Im gonna get it I think. If no one here said anything about the unloader valves, I would have never thought to check them.