Okay, to begin with I think you need a new mechanic. I do not trust a mechanic that recommends replacing parts because "they've been setting up for a long time." I have had these compressors set up for years with no ill effect.

She is right, however, about buying a NEW R4 compressor rather than a rebuilt.

You need to start by turning the engine off and turning the front part of the compressor clutch by hand to see if the compressor is free. If it turns freely, then you have a good starting point. Since there was pressure on the system when you cracked open the shrader valve, you have a relatively tight system.

The second reason to find another mechanic is that anyone in the business should know that when you have a low charge, the low pressure switch breaks contact to the compressor. This is to save the compressor because if there is no refrigerant, oil does not circulate through the compressor.

With the engine off and ignition switch and a/c controls on, remove the two connectors at the pressure switch behind the right headlight assembly near the sight glass. Short them together and listen for the clutch clicking. If the clutch clicks, you need to charge the system. Since there is still pressure on the system, you should be okay with not evacuating, just charge it up.

Even though R12 is expensive, it will be MUCH LESS expensive to charge, than to start replacing parts, flushing and all the other things you must do for a proper conversion. Converting a system is almost always penny wise and pound foolish.

Good luck,