2009 CA. DD15 Air System Mystery

Hi everyone, my name is Mike, i am administrating the garage for a transportation company.

I have a 2009 Cascadia DD15 #AG4081 which has some air system issue unknown to my head mechanic. The truck has recently (last month) got a new: Air dryer, Governor, 2 New check valves + we installed a new check valve on the wet tank due to missing it's own valve, we did that only because the air was coming back to the dryer and the governor would stop building air, that worked fine as my mechanic stated.

The new mystery came a few days ago where the truck, from it's 100-120psi in air tanks, after using the brake 2-3 times while driving, would drop down to 50psi and stop building air at all, the issue occurs randomly, we never know when it happens and why. Sometimes when starting the truck, it would not build up at all and then if shut off and left for a few minutes, the issue would disappear and start building again.

The compressor works, we checked it already, it can build up 120psi with no delays, works just fine. We did check for any air leaks and did not find any, water and soap spell tried, the curse is still present.

Could someone help understand the roots of this issue or if anyone has ever experienced/heard something like this, we would use any hints at this point.

As i understand, the air system has no computer systems working with it so there would be no fault codes to look for, if i am mistaken, please correct me and i'll upload the history of fault codes.

Thank you very much for attention and time.

Best regards

Mike