I am bumping this thread to update it...I succesfully "transfused" R12 from one car to another today. I didn't have to run the "donor" car, just hooked up the high pressure hose to the high pressure tap on the donor and the low pressure hose to the suction side of the recepient, then capped the "fill" line which had been attached to the vacuum pump after the recepient car was evacuated.

I purged a couple of seconds of R12 though the hose and opened the valves to let it start flowing...then I started the recepient car's compressor and "sucked out" enough R12 to run the sight glass without bubbles...took about 7-8 minutes I would say. Nice to know I can now work on the donor car's system, and not have wasted the R12 that was in there. Recycling at home!

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz