[rheavn]

By your description, you only have a leak when the engine is running? What does the supply guage do once you shut off the engine? Does it go to zero in just a few minutes? If it was leaking when the engine was off the 110V compressor would be running every minute or so. A compressor that runs every 1 1/2 hours indicates only very small air leaks in your auxiliary air system.

I agree with Michael. If air is not leaking with the engine off, I would be looking closely at the six packs(front and rear) and the ride height control valves with the engine running.