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

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