Yes, the R134a seals are backwards compatible.

As far as finding the leak, do you have any other plans? Can you get your hands on some nitrogen? (not nitrous oxide). If you have a friend who MIG welds, he might have a cylinder handy. Nitrogen is an inert gas, so you can charge your system with that (do NOT run it) and either listen, use the soapy water method, use some super-blu, or try the incense trick.

Now, while the seals might be backwards compatible, the refrigerant oils are not. If you've already made the switch to 134a, it would not be a good idea to load a PAG filled system with R-12. R-12 needs Ester oil. Either flush out all of the components seperately (take them all out) or stick with the R134a.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.