I've had some members contact me about this subject so I thought I would cover the correct procedure, as I have done literally hundreds of conversions with great success.
First Its important to know that the molecules of R134 are smaller than R12. This has a big effect on cooling capacity when just draining the R12, changing the charging port fittings and recharging with R 134.
The expansion valve. This valve is basically an oriface, that when the high pressure liquid refrigerant goes thru it, it becomes a low pressure gas inside the evapoator core. This chills the core which absorbs the heat of the air passing over it inside the car and carries it back to the compressor. Due to the fact that R134 molecules are smaller, they need to be squeezed tighter at the valve to produce the same or better results of R12. Virtually all replacement valves today are calibrated for R134.
The receiver/drier. Basically a small tank whose purpose is to trap small molecules of moisture in the system with a dessicant bag. This bag also will hold amounts of refrigerant oil and R12 , always should be replaced anytime major A/C work is performed. And virtually all replacements today are compatible with R134.
The refrigeration oil. This oil circulates thru the system to lubricate the compressor and expansion valve. R12 systems used a mineral based oil, which R134, due to its smaller molecules can not push thru the system. So on retrofit the compressor should be removed, drained of oil, and refilled with the proper amount of ester oil, a lighter synthetic refrigeration oil. Do not use PAG oil as its not compatible with trace amounts of mineral oil in the system.
O Rings. R12 systems used black, buytl rubber o rings. R134 use green HNBR orings, less suseptible to leaking. There cheap and easy to change. Be sure to lubricate them with ester oil prior to tightening the fittings.
Compressor, condenser, evaporator, hoses. All compatible with R134 provided they were in good working order prior to the retro fit. Change compressor oil.
Charging ports. It's not neccesary to buy those R134 adapters. In fact I rarely if ever use them, prefering the old school look.
Charging the system. After you have buttoned it all up the system needs to be pulled down to 30 inches with a vacumm pump for a minimum of 1 hour. Then close the gauges and see if it holds for 30 minutes. If so you should have no leaks. My preferred method of charging is to dead charge thru the high side port, engine off, till it won't take anymore. I do this with the refrigerant tank upside down (liquid). Gauges should read the outside (ambient ) air temp. Do this thru the high side only. Then i close the valve, turn the unit on and top it off thru the low side ( vapor, tank upright ) till I get the desired pressures/cooling. NOTE:: R134 runs at higher pressures than R12. If your sight glass is clear the system is overcharged.
Hope this helps out.
