Sounds like it could be one of your check valves. I had a similar problem and traced it back to the big check valve on the main brake booster line. Maybe isolate everything but the shut-off valve and see if it still does it.
Mine was completely wonky
Wouldn't shut off unless the hi or lo cool buttons were pressed in sequence
I just isolated each system one by one and finally got everything working.
I actually got my servo working great but it binds when it gets hot.
Cools off and it works, gets hot and the servo motor binds causing my amp to get hot and it stops working. Cools off and it's fine again...
I can loosen the screws that hold the "gear plate" (Under the top cover) with it hot and the motor starts working. My body is cracked where the screws go so maybe that has the housing distorted
I just read your whole saga and it sounds just like mine!
Pisser to get the servo part working and have the compressor/refrigerant part crap out