Please help us help you! Take a few minutes to edit your profile to include a signature line - similar to mine below - that includes at least your first name, home base city/state, and make/model of your coach.
By doing so, we can better refer you to parts sources, repair facilities and, as an added bonus, often a neighboring busnut!
What is the VIN on your coach? PD4106-XXXX?
Do you have the maintenance manual & it's supplement? There's a wealth of information in both, including air bag replacement procedures. The parts book also has a bunch of exploded drawings to help you, too. And an Operator's Manual will often answer "what is this for?" All four should be in your library, five if you include a DD 8V71 manual.
If you don't have it already, memorize this number: 1-888-262-2434. That's Luke @ US Coach in NJ. He's probably forgotten more about GMC buses than all the combined knowledge on the major bus bulletin boards. Strong supporter of our crazy hobby, knows the correct parts for the applications, reasonable prices and ships same day when you use plastic money. Support those who support us!
Working on a 4106 is not much worse than working on a GM car, only bigger.
Crib blocking of the coach body is much safer than using jack stands. Block on the main bulkheads on either side of the axles. A 20-ton air over hydraulic jack is extremely helpful.
Check the leveling valves also - you may have a bad one, not a bad air bellow.
FWIW & HTH. . .