I think my cylinder holds air after I put a new repair kit in it. But it's a decent idea nonetheless. I'll throw a gauge in after the first shutoff valve after the tank, and flip the valve. We'll see if the repair kit is working like I think (hope) it is.
And then I have to work on repairing the shop-air connection on the bus. The fitting mounted in the engine compartment doesn't work. Must be plugged somewhere along the line. Fixing that and using my air compressor will go a long way toward isolating any leaks on the rear suspension.