All GM vehicles with the rear auto-leveling shocks will sag after sitting for days. When you turn the key on though, after about 30 seconds the air compressor should kick in and auto-level the truck back up.

Once these shocks get to around 100k miles, they tend to fail and need to be replaced. Replacements are around $300 each, so $600 if you're going to DIY. It's a simple job - just 2 bolts, an electric connector, and the air hose.

 

What is the ELECTRIC button?

Is this some aftermarket switch that pumps air into the bags or has the Air Suspension Unit been modified?

 

Replace the air bags as they are not that expensive & its fairly easy. JUST keep in mind, the air pipes (plastic lines) that quick connect into the bags may be the area you are leaking at also.... others may have a way to check with soap & water misted. If I remember right you can remove the rear fender liners to squirt (I may be wrong- Im old and has been a few years since I have replaced mine.... (this is my 4th H2, done on all prior that had air suspension). If I remember correctly you can get both bags under a couple hundred dollars....