I've swapped a ton of GM systems from R12 to 134. Most of them worked just fine, and cooled just as well as the R12. The couple of vehicles that just didn't seem to cool as well were both very large. One was a Fleetwood Cadderack, and the other is my 'Burban w/rear air. The 'Burban will eventually cool down comfortably...it just takes a while to cool off the huge expanse of free space.
Everything else I've converted has cooled really well with 134.
Change the attumulator, the compressor oil, and the orifice, and you should be good to go. You really need to remove the compressor and drain as much oil out of it as you can...that's where a lot of the oil will be. Then put 1/2 of the new PAG/Ester oil in the comp., and half in the 'tumulator. Evacuate, charge, and chill.
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Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"