Have you found any clues regarding this?It can be parked for a few hours or day. It has been a gradual thing I think. I'm curious where to start. Could it be the "unloader" or a bad bag on the rear brakes or something blocking the lines? Could it be a leak, even though I don't hear a hiss? I'm trying to get an idea if you all have had a similar issue and I'll tear into it from there. I've had the truck about 5 years now and it would fully release the parking brake at 6 bar or less. I used to have to be careful because the 4-5 air shifter has always needed about 7 bar to work and it wouldn't shift. Now my "hold-up" is the parking or rear brakes.
I'm interested as my truck is doing the same. I can only release the hand brake at 7+ bar pressure. I found a couple of very small leaks from the rear but fixing those didn't fix the problem.