2002 Peterbilt 379 where is my air leak?

I have been fighting this forever. I cannot figure it out and Im lost on further diagnosis. Its a 2002 Peterbilt 379

The governor is kicking in and purging air every 5-6min. ONLY when the engine is running, either idle or at speeding going down the road. I have gone through and fixed every leak I could find so far.

Airbags
Suspension leveling valve
Cab leveling valve
Governor
Check valve on passenger side air tank
Cleaned air dryer purge valve and replaced the o-ring. Air dryer is an approx 3yr old Bendix Ad-Sp and not cheap.

Now this is what throws me off. When the governor kicks in and purges the air at 130psi, if I leave the truck idling the purge valve will continue to bleed air to 120psi and the it stops. Takes about 20sec. Then it will slowly lead down to approx 110 psi and start building pressure again.

Now, if I shut the truck off before the gov kicks in say 125psi and leave the truck off, the truck will hold 125psi for hours.

Like Ive said, the only part I haven't replaced is the dryer.

What am I missing? What can I do to further diagnose this before replacing the dryer?

 

Its an AD-SP. The gov is on the compressor

 

I've checked both lines coming off the gov. The neither the line from the wet tank to the gov is leaking, nor the line from the dryer to the gov.

Is there a check valve in the air dryer?

 

Haven't replaced the compressor. Output seems good, Ive pulled the supply line and no air is leaking from there that I can feel or hear.

 

I want to say Ive replaced it in the past. Ive replaced the fan clutch and the line within the last 2-2 1/2yrs.

But I will double check it in the morning.

Edit: It seems the the solenoid and clutch are all 1 unit. So its been replaced in the recent past with a brand new Caterpillar part.