15 prostar. Cummins isx15 425. Loosing air while parked at night. I can have full pressure in tanks and set the brakes on both truck and trailer. Shut engine off and wishing 2 hours it's beeping at me. Overnight pressure in both tanks will go as low as 30 pounds. It does this with or without a trailer. And I'll be ###### if I can hear or find a leak anywhere. I did change the compressor ( for other reasons) and I changed the brake valve on the firewall in hopes that would solve the problem which it did not