Here is a quick and easy check after you build up air pressure and turn off the engine.. Fill a spray bottle with water and dish washing soap. Shake well, and spray on all the parts everyone else has been telling you about. There is a valve and pressure regulator that controls air going to the small air bags. They are known to be potential leak points. Spray them down good and look for bubbles. Spray every connection you can find that is related to the belt tension system. Spray down the air bags and their fittings. If you do not find a leak there, then it gets a little harder. You have to figure out what else is on the aux air system and start spraying every air fitting you can find. It is possible to get things to the point where the aux air will stay up for weeks at a time. Just takes time and patience to find all the leaks. I found my last air leak in the toilet. I have a toilet that is flushed by air pressure and there was a fitting in the back of the toilet that had a very small leak.

Good luck.