I am having a similar problem with my 2002 Envoy SLT. I now have over 100,000 miles. My problem started around 80,000+ miles. The rear of my Envoy will squat. As soon as I start the ignition, the air compressor will inflate the air bags and the ride is fine. I have taken it to the dealership several times and left it all day but they can not find what is wrong with it. Ironically, it will never deflate when it is at the dealership. I even took pictures to prove I was not making this up. Now, there is 1 other vehicle that is doing the same thing. Does anybody know what is causing this to happen? They put in a new pumb but it is still happening.