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!

Very cool, good for you. I'll have to try this someday...

I have used Cressida's method for years on refrigeration systems using R-12 and R-22 with close to 100% success.
Freeze the big empty R-12 cylinder. Pull a vaccum if you have the equipment.
Hook the high side and low side on the donor A/C system to the manifold Hi and Low sides. The third hose will go to the cold cylinder.
Befor connecting the hose to the cylinder slowly open the valve to purge the air from both sides of the hoses. Then connect the last hose
With all the hoses snug the open both sides of the donor valves. The compressor does not have to be running on the donor system.
As the frozen cylinder warms up or in about 6 minutes on an 80 degree day
the donor system will be about 95% empty.

Use an R-12 cylinder that has not been contaminated with other types of freon or left open with air let into it.
The refrigerate taken from the old system may be contaminated, maybe not.
Wear safety glasses, gloves, etc.
Let us know
Al