Air building and dropping!

So it is working right now?

 

^^^^^^Plus One to This^^^^^^^^

Get your Mechanic to plug his Shop Air into the Wet tank & Get System up to 120psi & listen for Leaks.!!
Try with Parking Brake both on & Off ( to isolate which Circuit leak is in) You May need to get either Your (Or Mechanics) Kids/Grandkids to listen as their Ears will be a bit more sensitive than older Guys...

 

I would assume that the mechanic already did this and ruled out split hoses and bad parking brake chambers.

I had a truck with a split hose that would only lose air when the truck was moving, rear parking brake hose. It was found by accident when the driver was under the truck trying to find it, he grabbed the hose and moved it which produced the air, would not leak when it was parked.

 

Yes its all just a process of elimination really, losing air, is the hose attached, is there a crack in one of the airlines, if not, hows the air compressor, etc,etc.