[Davidf]

Interesting timing on this thread. I purchased a 71 this past Saturday with factory a/c. The a/c is non operable, but all there and seemingly original equipment. It is also obvious that 134a was used as the low side valve is of the 134a type (adapter fitting). My plan is to go back to R12.

I live in Austin, TX where summer time temps reach 100+ very regularly. In my past experience with converting R12 systems to R134a it just doesn't cut it in Austin. The reason is mostly the condenser....R134a is less efficient in eliminating latent heat than R12. Thus you need a large condenser to shed that latent heat from the R134a refrigerant. If you live in cooler climates then a conversion usually performs fine (i.e. Chico, California).

Also, I suspect the original factory components are robust enough the handle the higher operating pressure that you get from R134a. The hoses will be thoroughly impregnated with oil and thus probably wont leak out the smaller R134a molecules. The compressors seals may be an issue and I worry about oil contamination. Probably a good idea to have the compressor rebuilt and tell the shop that you plan on converting to R134a. I will disassemble my system and flush all lines, hoses, evaporator, and condenser before switching back to R12.

I know it may seem crazy to switch back to R12, but I have a 30# cylinder of R12 sitting in my garage (about 2/3 full) with no vehicle to use it on. And, I am a certified a/c mechanic (got it online many years ago) so I can buy R12 from the auto parts stores. R12 simply works better than R134a.

Hope this helps...

Edit: Almost forgot...you MUST change out the receiver/drier...oil contamination if you don't. All the other components should work just fine after a thorough flush. And, freezing of the evaporator should be a non issue since the system wont cool as well as with factory R12. So, if it didn't freeze up with R12, it surely wont with R134a.