I have a 1998 Navigator, 4Wheel Airbags. The rear bags are bottomed out. The front never changes.?

The Compressor Runs, Just Puts a very small amount of air out. The front bags Havent changed since I bought the vehicle a week ago, But the rear bags wont pump up. I can turn the key on, and the Solenoids click, An the compressor runs, Just wont pump up. Anyone tell me any problems or where to start on this?