I am sure someone here has done it before and can answer my questions.
With a correct charge does the 134a cool as well as the R12 did?
What pressures can I expect on the low and high side?
What all is changed with a propper conversion? An amature conversion? Expansion valve/ orifice tube, Oil, Lines, condensor, evaporator, compressor? Or only what caused the problem in the first place then the propper amount of refrigerant ?
porksboy wrote: With a correct charge does the 134a cool as well as the R12 did?
Absolutely not.
The couple I have done have been amateur to say the least. They consisted of the screw on valves, a vacuum, and refill with oil and 134. They worked much better than an empty R12 system. 
The pros change all kinds of stuff. Receiver/drier, orifices, ect. I'm too cheap to put that kind of money in a $300 car.
What pressures did you look for?
Low side around 40, High side around 150.
The real problem with this conversion is that the two systems use different oil. If you don't get all the oil out before re-charging, your compressor WILL fail. The only is how long will it take. The reason you see big differences between amatuer and professional conversions is due to differing expectations. As Toyman states, he wasn't willing to change all the parts, but he has a/c now when he didn't before. Techincally, the compressor should be changed to a 134 type so that it can provide the flow and pressures needed to run 134 effectively. The expansion valve is changed for the same reason. The drier gets changed because it is virtually impossible to flush all the oil out of it. O-rings should be changed to a Buna-N type to insure sealing with 134, since the molecules are smaller than R-12.
Recovering and evacuation will not by itself remove all the oil from the system, it must be flushed, preferably with refrigerant. And generally speaking, the condenser and evaporator in a 134 system will be larger for the same ouput as an R-12 system. System pressures depend on many things like temperature, humidity, engine speed, etc..
and cannot be accurately estimated. Your car will have a capacity spec, just charge with that amount and you should be good. All else being equal the pressures will be higher than with R-12.
The only thing that is mandatory in doing the conversion is to flush, change the oil and drier, evacuate, add the appropriate charge fittings and charge with 134. Everything else will improve performance over not doing it.
Just keep in mind: Low side should be approximately the temperature in F you want air coming out of the evaporator. As low as you can get without icing or over-cycling the compressor.
Thats the un-scientific way to charge A/C with R134.
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