Rear Air Suspension
 

Okay, someone has had to have the same issue. About once a month I will got to my Hummer after its been sitting for awhile (sometimes over night) and it will be sitting ass low...the air suspension drained out. As soon as I start it up, the pump kicks in and its back to normal. Then it will be fine for a month or so. So can anyone tell me what the heck is going on and how do I fix it.

Sounds like an air bag leaking, air bags can act strange and some days leak and other times not leak. I would get under there with a flashlight and a spray bottle with some real soapy water and see if you can find the leak by watching for bubbles.

Air Suspension
 

Any idea what that would set me back to replace?