If you do an evaporator replacement, the refrigerant will be the LEAST expensive part of the bill. Stick with R12. Due to "Black Forest" condensor design and size, our cars don't make close friendships with R134a. The hygroscopic nature of the oils (PAG or ester) for 134a makes acid production more likely and evap. life much shorter. I stopped arguing the efficiency issue long ago ( too many non-scientific opinions), but I will expound the acid issue. I hope it's just an "O" ring gone bad, though, and not the evap.