Truck losing air pressure when parked,can’t find the leak

Earlier today I was going to get my first load of sand and I drove unto the scale/silo and set my brakes.A minute or two into the loading process the low air light comes on.I finish getting loaded and drive over to the area where we can roll our tarps up and started looking for a leak with the truck off but found nothing.I ended up idling up t’ill I’m at 90psi and start driving down the rural road leading away from the sand plant.The highest PSI I got was 100.When I made it to the site I got in line behind a bunch of trucks and every time a truck moved forward I released the brakes and the low air light would come on.Just finished unloading and pulled off the site to an empty site down the road and checked everything again with the truck off.Also with the wheels chocked and brakes released.And with the key in the on position and found nothing.What could be causing this?

 

2003 International 9900 with 2004 Cummins CM870.A mechanic looked at it every put his hand over the compressor outlet and he was able to hold it easily so he says it’s the compressor.

 

I just saw this,but I installed the compressor and a new governor and it all seems to be working fine now.