Just swapped out my air dryer and with the truck off it doesn't leak air but with the engine on the air pressure steadily goes down to 100 psi in only the primary tank then it recharges and repeats the cycle I checked for leaks with the engine on and off and I'm kinda stumped.
charge up your tanks, turn off the engine but leave the ignition on, listen for leaks around your fan clutch
I'm suspecting that is what's leaking because I got a 04 volvo with a cm870 which shuts off the air flow to the fan clutch when you key off and I could kinda hear it before it cuts it off I will close off the hose and test again.
After testing fan clutch is leaking air and truck holds air when running with the hose plugged off