Change over to 134a it is easyer and more convienent and it is available anywhere. When 134a first came out this was difficult now with the ether oil that is sold whereever you buy 134 it is simple.
You will need to change the shreader valve to a 134 coupling after the system has been properly drained of R12. (Be sure to remove the old shrerader valve.) Vacummn the system, the first can will be the ether oil, then followed with 134. Be certain to only filll it to 80% capacity of the R12.
Enjoy cooling days.