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.