I don't know why I didn't notice your request until today. Sorry about the delay.
Full extension would be with the truck body high and the wheels drooping as far as they will go. This needs to be done because part of the air bags are hidden against their seats, and the leaks might be hidden. The problem with this is the system will reduce the air pressure in the system because it sees the body as high, and the lower air pressure will tend to reduce any leaks.
From your problem description, I'd guess there is a valve that is not sealing in your compressor package. Unfortunately GM only sells these junk compressors as a unit. They get a ton of money for what amounts to a cheap imported compressor, a couple air solenoids and a cheap electronics package.
Send me a e-mail at email@example.com
and I'll reply with what little documentation I have about the air suspension.
All standard disclaimers apply.
-- Dan Meyer