... I'm not a pencil neck engineer ... but you might be physically able to evacuate the R-12 and add R-134a ... but it will not work correctly and will not last long.
R-134A Ac units are physically 30% larger ...which means you need larger coils. The hoses have higher ratings and the seals are of different composition .
How about buy R-12 and just fix the existing system ... that's what I did and my car is ice cold inside in the summer ?
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