05 Pete 379 - parking brake issue - "bleeds" air if Air Pressure is below 100 PSI

It's been a month and $3000 later and I still cant seem to find anyone who can resolve this issue. I have a 2005 Pete 379 that started to have brake issues. First it was building 150 PSI constant or just not build air pressure at all... new Air Dryer filter and Governor... Then on came the issue where the parking brake "bleeds" air if the air pressure is below 100 PSI and will continue to "bleed" air until the air pressure drop below 60 psi and the knob pops out. It doesnt do this all the time... it is random... To get around this issue, you apply parking brake, build air pressure to 120 psi and release.

Let me try to explain what I mean by "bleeding" air. In General, when you release the emergency or trailer brake, you get a slight "bleed" or "release" of air for several seconds and then it stops... make sense, hopefully.

I've taken it to 2 different Peterbilt shops... they both replaced the entire valve assembly, claiming that it was faulty. I took it to a service shop here in the town I'm working in and he isnt sure, as his first step would be to replace the valve assembly...

Anyone have any ideas? I can still operate the vehicle safely... its just a pain in the ace to deal with.

 

I an pulling a trailer and it is only the parking knob.

 

I had both the Pete service and the local truck repair check the compressor... they all 3 said it was functioning properly.

 

I will go thru the truck and trailer and see if I can locate any sort of leak. The truck does lose air... if it's parked for 3 or 4 days

 

The issue originally started when I was bob-tailin back to the shop and stopped for fuel, but I will give it this a try