Mack will only build air when pumped down to nearly 5psi

Hello all. Scratching my head on this one. I have a 1986 Mack Superliner triaxle that will not build air unless the system is pumped down to nearly empty. It builds to 125psi as it should. The compressor has less than 30k. The unloader valve is clean and I have replaced the governor twice. It also has a brand new air dryer. The lines appear to be clear and yet this keeps happening. Any thoughts are appreciated.

 

I disconnected the signal line. The compressor pumps just fine. I tried a third governor and the same problem occurred. All the lines blow clear.
I did just find something out… I was away while the truck went out for a new frame. Turns out the builder replaced some airlines between the tanks. Maybe he hooked up some lines incorrectly? Even so, the lines going to the governor are in the correct location.