Solenoid Exhaust Not Allowing Build Up

Honestly, that’s totally strange. I won’t say no one does it, but the only cases I knew of until now are when teams accidentally left the vent plug open. You should have led with it.

connect another solenoid, even used one, and check if the nature of the leak changes

I suspect other posters are on track with the pilot pressure issue, but one other thing to check is that your manual override buttons aren’t rotated into a detenting position where they’re holding the solenoid in position, regardless of air pressure. The little notches in the blue buttons should be aligned with the body of the valve, lengthwise, to allow the pilot solenoid to do the switching.

If your issue with leaks/switching stops once you’ve built up 22 PSI in your system, then this is almost definitely not the problem, but it is something to be aware of.