I have a 2005 Hummer H2 with air suspension issues. The vehicle squats in the back after sitting for a while. The compressor works and the air springs reinflate after the vehicle has run for a minute or two.. With that said, I assumed that the air springs were both bad so I change them. The old springs were cracked and weather checked. Not how old they were. I started the vehicle and the compressor aired up both springs. I let the vehicle sit for a couple of hours and when I returned, it was squatting in the rear as before. There is no audible air leaks. Anyone ever have this problem and know the fix?

 

I did have a problem on the driver side rear Bag like you. It turned out that when I was off roading I had the *** end jacked up. Sand got caught around the base that the bag sits on. When I deflated the bag back to normal position the sand rubbed and punctured the bag. I had it replaced and all was ok.

If you search here you will find info on the bag controls. There are leveling arms that monitor the height and talk to a Computer somewhere on the rig. I remember reading that the height adjustment is done with a tech tool where programing is sent to one or two modules.

The guys here will jump in and explain the system. I have an 09 H2 so not sure about yours. It's come up here before so hang in there. You'll get the full story as soon as as they pick up on your post.

 

@ BMoves your AS unit should be a twin compressor system the same as my 05 and should look like below:-



This is the air control solenoid or "Air Level Control Solenoid" GM 88982408, hard to find.



99% this is an air-leak rather than electronics, but finding it could be tricky because both bags are loosing air quite rapidly suggesting a common leak which could be a cracked solenoid chamber (worst case)

The standard method is to apply soapy around the ends of the air-pipes, airbags & solenoid connectors, solenoid body and check for bubbles.

A common problem are the connectors where the air-lines push-fit into solenoid, these tend to corrode and leak, but can be cleaned (use no grease or oil)
All the collars (2x 6mm and 8mm) can be gently lifted out with a flat blade and down each hole are 2 O-Rings which easily be removed. Clean everything with IPA, bores, collars, O-rings, pipes etc. this may help.

In the jpg on the RHS of the solenoid is a big brass pressure sensor that screws into it, this also has an O-Ring, it's worth checking and replacing if it leaks.

is about servicing the solenoid, it might be worth a watch to see what's involved.

Note the large 8mm pipe on the LHS of the solenoid is unlikely to be a problem, once the 2 solenoid valves are off the air is trapped, it can only escape from the places mentioned above

 

Yup, you have a slow leak!

To add to what oceanbrave said, lift your vehicle to highest setting. Spray soapy water on the air bags, and look for micro perforations. Let the soap sit on for at least 30 seconds, and the look for extremely white microfoam bubbles.

Also spray top of shock connectors, and observe with a mirror.

Likely a bag or connector, and less likely a solenoid. Not a leveling arm.
If connector, examine end of hose. If squashed or leaky, cut off a 1/4 inch and test again. Easier than re-re of the quick-connect

If you have an onboard on-demand air nipple fitting, check fitting and hose to it from the compressors.