Having a discussion about a friends coach air pressure goes from 128-115 almost instant after the compressor shuts down. Both tanks are showing the lower air pressure, pressure stays around the 115 mark for a while and slowly goes down to the cut in pressure around 107. My coach when it cuts out at around 128 will slowly go down from there until compressor cuts in.
We are trying to figure out what is causing this which brings up my question as to how the pop off valve on the air dryer works. Does the pop off stay open until the compressor starts back up or does it open on compressor shutdown and then closes back up?
Sounds like he may have two issues:
Check valve(s) not working. Irrespective of a malfunction of the dryer/purge valve, the check valve(s) should keep air from going from tanks back to dryer.
Purge valve sticking open-- it should only open momentarily when cut-out pressure is reached, then close (remember that "sneeze" from the dryer-- it is close to instantaneous.
That is what I was thinking but I was not sure exactly how long the purge valve stayed open.
It is a "sneeze"-- no longer.
Learning a lot on this project. Believe I have most of the external leaks under control--lot of bubble bath. Now looking at internal check valve or PPV valve leaks--harder to identify. No check valve between wet tank and air dryer--only one is on the Cummins air line. Believe front DOT tank check valve is leaking back, rear tank holds at 52 PSI. As mentioned, Cummins compressor cut off at about 125lbs but DOTs quickly settle to about 115lbs. Suspect air dryer is OEM--thinking about just replacing the whole assembly. Ref attached, any additional thoughts would be appreciated.
Strange there is no check valve at the wet tank inlet port. I always assumed that was standard air line plumbing practice. There is an internal check valve on most (if not all) air dryers. It is part of the dryer delivery port assembly and should be replaceable. See "typical" AD-9 dryer schematics below. I realize you probably have a different model dryer on your 2015 model coach.
Bendix® AD-9™ Air Dryer (https://www.yumpu.com/en/document/read/11674226/bendixr-ad-9tm-air-dryer)
PS: I took the liberty of adding your air schematic to our Forum library. THANX!
https://www.foreforums.com/index.php?action=media;sa=item;in=5020#viewitem