1993 Cadillac Deville auto-leveling system

Hey all, I am having a odd problem with my 1993 cadillac DeVille. While driving my vehicle the rear of the car loses air and and drops so low that it rubs the tires. To correct the problem I must stop the car, turn it off and take the key out, when I restart the car the rear pumps back up to a normal ride height. It seems to de-air at random, the vehicle will sit in the garage all night without losing air, but out on the road it drops frequently and has to be attended to. Any help or knowledge about what could be causing this would be greatly appreciated, Thanks all