I have to disagree, to a point
Some cars convert quite well to R134a
These old cars almost always have a leak . . . line, hose, compressor, evaporator, etc.
Replace visibly leaking components
replace receiver-drier. The new receiver-drier will be compatible with R134a. The old one from 1982 will not work
make sure your pop-off valve and pressure switches can handle the slightly higher R134a pressures
make sure your condenser fans are working properly. Make sure the radiator fan is working correctly. make sure you’re not missing any fan shrouds or air dams
evacuate (vacuum) at LEAST 30 minutes. After the timer stops, check if the system is holding vacuum. If not, you’ve still got a leak, which you haven’t found yet.
charge only 80% of the original charge. If the old label said 3lbs R12, then you charge only 2.4lbs R134a, if my math is correct
When you’ve got the car idling with the compressor engaged and operating on R134a, suck in some uv dye through the low side port. makes it much easier to spot leaks later on
cover up the old R12 label, and affix one which reflects the date of conversion, amount of refrigerant, amount of oil, who did the conversion, etc.
I’d rather convert to R134a, versus charging with R12, which may be counterfeit and possibly dangerous, not to mention it can be ridiculously expensive
as mentioned, not all of the older design compressors work well with R134a