need a lesson in A/C

I will tell you what I know about mobile a/c. Most people that work on cars do not understand how it works. If the system was made for R12 it will work much better with R12 in it, assuming it can be made to work. R12 is still available. Those old reciprocating compressors do not like R134a. Old hoses do not like R134a and they can leak badly. Condensers made for R12 do not generally remove enough heat when using R134a. Those cans or a/c fix you buy at the parts store do not contain R134a in general, since that requires a MACS license to purchase. Most parts store a/c fixit cans have a hydrocarbon compound in them, similar to propane. If you to have a shop to work on the a/c system, ask them if they have a micron gauge to use when evacuating the system. If they do, they probably know what they are doing.

I would consider this a all or nothing proposition. If anything needs to be replaced other than a drier, I would junk it all and convert the system to a full system with a Sanden compressor and R134a. Before doing anything I would fill the system with dry nitrogen to check for any gross leaks. If it passes, then vacuum the system, preferably under 50 microns but at least under 500 microns, and if it holds that for an hour it will hold refrigerant.

That's it. That's all I got. I might know more but I can't remember most of what I know, and half of what I can remember is wrong. #-o