04 GMC Envoy has been lowering in the rear and not going back up anymore?

Sounds like the pump may not be coming on which could be caused by a bad fuse, faulty pump, or faulty wiring. If you can hear the pump running there may be a leak/hole in the piping or in the air bag itself.

What happens to the system is a small leak occurs and causes the compressor to overwork and fail........I would diagnose for a leak after the new compressor is installed......If you have problems try my shop......good luck!