I had the same thing happening on my 2004 a couple years back. There is a plastic connecting piece that is between the tube from the air pump that goes to the airbag. I believe it may even act as a check valve(not sure tho). That plastic piece connects the 1/4" plastic tube to the 1/4" plastic tube on the airbag, mine was broken and leaking air. The piece can be found behind the plastic, behind the tire. It is about $20 at the dealership, if you do it yourself. You could have a busted airbag but I would look into the fitting first. When mine got really bad every time I would try and air the suspension up I could hear it leaking standing outside the vehicle. I also remember my air compressor/pump was always running, and that makes sense as it was trying to keep the suspension aired up while leaking at the same time.
Not positive this is your problem, but I know that can be frustrating as hell!