[77newell]

Let's split th e system into two major systems to start: there are two gauges on the dash; the dual needle brake gauge and the single needle supply gauge. If the supply drops and neither of the brake needles drops with it then you can eliminate the brake system. If one of the brake needles drops with the supply until the supply hits 70 and then the brake needle continues to fall but the service holds pretty well then you need to check the brake system for leaks.

Assuming it's the service needle that's falling but not the brake needles, then we need to split the rest of the system in half if possible. On my 93 coach there are a pair of ball valves on the bulkhead just ahead of the drive axle. Closing those valves separates the leveling system from all the other "service" uses. The pressure gauge at the rear 6-pack shows the pressure on the leveling half and the dash supply gauge shows the pressure for everything else. Seeing which one drops the quickest tells you where to look..

Given that the rear drops when pressure drops, but the rear stays up when in leveling mode suggests the leak is somewhere between the supply to the 6-pack and the 6-pack and includes the HCV's since the HCV's are not isolated from the supply air when in leveling mode, their output is only isolated from the airbags.