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Rocket.... you're only going to lose 15% of your cooling capacity by using 134 over R12. I found on my Grand Nat a couple years ago, I lost 4* at the vents when I converted to R134.... but that's your choice. Just remember..it's better to be cool with the windows up in July than to be cool sitting in 90* rush hour traffic (with the windows down)! Good Luck!
Rocket.... the key to optimum performance using R134 is to change the orifice tube and the drier when you change the compressor. I always pull a good -28 to -30 vacumn and let it sit to make sure it holds. While it's true R134 isn't as efficient as R12, I dont buy into the fact that a 134 system won't cool efficiently..... I cant recall which compressor your running, i have to guess a R4 because their 5x more prone to failure..... but if your running a Nippen-Denso, you should be able to get your system back to cooling without breaking the bank. I'm in the process of making an AC video for youtube....I'll post it on here....