Air ride is sagging out in the rear

What would cause the rear shocks of my 89 Lincoln Mark VII to (quickly) deflate at times and for no apparent reason? As an example, one day the car sat level for 6 hours and then I drove it 2 minutes to a gas station and filled up. While I was in the kiosk the rear end dropped all the way down and once I started it the compressor brought it back up. Another time, while sitting in the car with the radio on, the back end suddenly dropped, then it did again and again, until the suspension bottomed out. I have also noticed the rear left rises higher than the rear right but I have not determined if it stays that way or levels out while driving. My new battery is dead but that may be from something else-the car has been sitting and there may be an yet undetermined draw.