Adam,
As a preventive maintenance measure, since our bus is at the 10 year mark, we have decided to replace the six rear air bags, 5 port Norgren valves (2), 3 port Norgren valves (3) and 2 ride height valves.
While we're in there, the 2 drive axle and 2 tag axle brake chambers will also be replaced.
Luckily, the shocks appear to have been changed.
The front bags were changed last spring.
In my opinion, at this stage of life for our bus it makes sense to simply replace a many components as are realistic to access at a time and try to eliminate chasing problems down the road.
Oh, and Joe will be performing the surgery.....
I'll keep ya posted as to how the "shotgun" approach worked out.