Suction pressure indicates he is NOT getting "mega" cooling. The thing is, the TXV or whatever is causing the problem is/ may not be completely shut off, it's just leaking a little. This explains the low suction, with not much heat load, and the low head, considering the shop is not all that warm.
But as I said earlier, don't discount that it could be water in the system, which TYPICALLY will work for a minute or so, then freeze up at the TXV, causing a restriction/ plug
I'm no expert on all the hardware used on cars over the years, but I spent a few years working on HVAC/R
Whenever you get a system that has been left open, and or not properly evacuated, filters replaced, in the case of vehicular AC a "sweep test charge" is a good idea, and don't discount that if you FOR SURE do not know the origin of the refrigerant you are using --off brand mixtures, or "recovered" or even a "used bottle" it might just be contaminated right out of the can.
It's really too bad that these compressors don't have proper service valves like the Ferds typically did, because it makes servicing/ testing these systems a lot easier.
Service valves have a "double seat." "Normal operation" the valve is "back seated" which closes the service (gauge) port. You hook up the gauge, "crack" the back seat open (CW) and now you have access. For certain tests, it's handy to be able to shut off the suction or discharge valve (front seated) which you cannot do on the Mopars.
"REAL" refrigeration systems typically have valves on the compressor, as well as the receiver outlet. This allows you to service a leaky evap, or change a compressor or TXV without pulling the refrigerant out of the system.
