2013 Maxxforce 11 air issue

Have been trying to locate an air leak for some time now. Many bottles of soapy water. Just discovered air pressure drops from 135 to 120 three times as fast when the brakes are released and the engine off as when they are applied with engine off. Compressor runs more than it should, loads every couple minutes, and air will drop to under 60 overnight. Before I start tearing into the rear brakes, any suggestions where to start or how to do a more involved test?

 

I don’t know if it’s just a CA thing, but compressors here all have to cut off at 135. Mine kicks back on at around 120

 

A great mechanic I use sometimes had to go to a truck last week that was red tagged cuz his governor needed didn’t cut out at 135. I replaced this governor a couple months ago and it’s set at 135. Sure smells like CA.