Kenworth T600 Losing air pressure

I have a Kenworth T600 builds 120 lbs purges then starts to drop pressure. This is while the truck is sitting, I turn the truck off and I cant hear anything. When you press on the brake pedal you will loose 15-20 lbs but will not loose anything with the pedal held down it will hold pressure (might loose 1-2lbs) still cant hear a leak. So my driver will go from 120 to 60lbs when he has press the pedal a couple times to brake. Brakes have been adjusted and work.

So compressor builds to 120 and starts to drop while dropping if you need the brake your gonna loose 15lbs every time you press the pedal and will give low warning signal but will build back up

Any Ideas?

 

Found 2 loose fittings in secondary tank and gov was sticking. So it would build up to 120 purge and start leaking down to 90 and would take for ever to build back up. After I changed it, goes to 120 and stays there.