I forget exactly how this works, but IIRC, R-134a is a drop-in replacement compatible with R-12 systems. You might want to double-check this with your mechanic- it might be R-(something else) that I'm thinking of. The only drawback to this is reduced cooling capacity of the system. Last I checked though, an R-12 service cost something like a couple hundred bucks. I doubt an R-134a charge would cost anywhere close to that much.
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