| previously I simply had my system vacuumed out and R134a put in (new drier). And honestly it didn't work very well either (it was ok in the car) and I found out they ruined the threads on high pressure line and now I have to get a new or have this one fixed which is another set back.. I just came from visiting with a company who specializes in AC systems and I was getting a little lesson on R12 vs R134a system. Basically R12 use a serpentine system for cooling in the evap, and condenser. While R134a uses a Parallel Pass. The end result is that R134a works inefficiently in a R12 system (this my understanding), unless you convert condenser and evap over. Using Coz's pic
Here is the R12 Evap
and here is the R134a Evap they quiet different as show here..
 I just got in a 93 EVAP core to try this was before I learned about the true difference in the performance of the EVAP cores. The 93 at least as a Thermo control amplifier to shut off AC system (you have to wire into the ECU For R12 cars with STVs) when the EVAP freezes up. Also because of the increased head pressure on the converted R12 cars you may want to switch your pressure switch to a newer one which is a high and low pressure switch, R12 cars had only Low Pressure switches (thanks to AliaZ for the information), if you use the newer style you will want to get a new harness I was able to score both a switch and connector from CZP. I think the bottom line is that if we can get R12 stay with R12 the system was made to run with it, yes it costs more to recharge but then again an Evap/Condenser/binary-switch and all the time costs a lot as well. I will say that I do have my orginal EVAP box and besides drilling out the screws (and retapping) gutting it took nothing but a few secs. There is a good write up here..
[ http://z32.wikispaces.com/Suction+Throttle+Valve ]
Just remmeber if you do this you may want to wire in the Thermo control amplfier to cut off the AC if EVAP freezes overl.
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