2015 Freightliner Cascadia Secondary Air Leaks

2015 Freightliner Cascadia equipped with Eaton 10 speed and Detroit DD15.

Truck is losing air pressure on the secondary side and filling to purge about every 1m30s. It was about every 30s but I replaced the check valve in the air dryer and that seems to have slowed it down.

I have purchased a governor and a purge valve kit and plan to replace those as soon as I have the downtime but this is driving me mad.

Truck builds air pressure just fine. It leaks down when the engine is not running--I would say about 2h and the tanks read 0psi. But when engine is running, either at idle or driving, the secondary tank drops from cut out to cut in between 30s to 1m. The primary system tends to hold pressure just fine.

I have scoured the secondary system looking for leaks and can't find any externally. Replaced the check valve for the primary tanks and replaced the Pressure Protection Valve on the secondary tank (PPV was leaking). The check valve for the secondary tank is a non-serviceable internal valve between the wet and secondary and I originally was thinking this valve could be bad and backbleeding into the wet tank?

I drained the wet tank, however, and both systems seemed to hold about 100psi. Could the secondary tank's check valve be "weak" and not necessarily bad? As in, when the pressure difference on either side is 0psi vs 100psi it would hold but maybe not when that pressure difference is a little more stable at, say, 90psi vs 125psi?

At this point, I'm open to any and all answers. I'm really thinking this is a supply issue and not a secondary issue, but I have been totally wrong before.

 

With engine off and tanks full:
1. Parking brake set yields loss of about 7psi over 1m.
2. Parking brake released yields loss of about 5psi over 1m, once settled.
3. Foot brake applied yields loss of no more than 2psi over 1m.
NOTE: Both tanks seem to lose pressure in tandem.

With engine idling and tanks full:
4. Parking brake set yields loss of 10psi.
5. Parking brake released yields loss of about 10psi over 1m, once settled.
6. Foot brake applied yields loss of about 25psi over 1m.
NOTE: Primary loses slight pressure while Secondary drops noticeably.

 

I've gone over the chassis and listened and sprayed for leaks and have replaced fittings where even minute leaks were found. No valves are exhausting air.

 

Not leaking through the service brake system. Not backbleeding through the wet tank. Idling just now, system builds to cutout and then leaks to cutin in just 14s. 30psi lost in 14s. I'm clueless.