AC Compressor Question

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Simple question: How can I tell if my A6 compressor is good?

Detailed question: I'm going through the AC on my 72 c10 with all factory air.
I've replaced all vacuum lines, cleaned out every orifice and everything appears to be functioning correctly. (compressor clutch kicks on, kick panel vent opens as designed, blowing through the correct vents, etc.) The system is still R12 and before I make the R12 investment , I want to make sure my compressor is functioning as it should.

I placed a set of gauges on the unit and while running both Hi and Low read zero. I also used a HVAC sniffer (very cool tool!) and found no leaks in the system. (I have a vacuum pump and will also pull a vacuum to see if I can identify a leak) Last question, any thoughts on a rebuilt A6 (RockAuto $140) vs a new Delco ($400) I understand the rebuilt A6 compressors tend to leak at the shaft because most have been rebuilt many times.