I've done lot of R134a conversions. They really aren't that good. Here are a few facts to ponder.
1. R12 is a better refrigerant and operates at lower pressures. You will never get an R12 system to be as cold as it should be by using R134a. Partly because of the poorer refrigerant properties and partly because the expansion valves/orifice tubes for R12 systems are not design for the proper pressures to operate R134a. You may be able to get a different orifice tube but the expansion valves used in Mopars would have to be tuned to the R134a. I don't know of anyone who sells a replacement that is properly tuned for R134a.
2. Converting an R12 system to R134a is a temporary fix since the R12 systems are lower pressure designs. The higher operating pressures of R134a will eventually cause leaks. Even with new hoses the systems will not last as long before a leak will occur. Part of this is because R134a systems typically have 2 pressure sensors (1 low and 1 high) and a relief valve. Most R12 systems have 1 low pressure sensor only. At normal charging you are turning 800 to 2000 rpms which is not indicative to a normal driving experience. Imagine stomping on the throttle to pass somebody with the A/C on. Modern cars will either electronically detect a full throttle and temporarily cut off the compressor or when the pressure spikes the High pressure switch cuts off the compressor for you. An R12 system will simply spike to over 300PSI on the high side without much trouble. Easily over 100psi more than R12 will do and was deigned for. It can only take that so long before it leaks.
3. R134a requires approx 80% of the volume of R12. Keep this in mind while charging. Overcharging will result in very high pressures. In modern R134a systems there is usually a high pressure vent to release excess pressure. Those are not usually on R12 systems.
4. R12 uses mineral oil R134 uses ester or PAG oil. Two factors to consider about the oil. Compatibility with the refrigerant and compatibility with the other oils in a conversion. The oil is carried by the refrigerant through the system to lubricate it. R134a will not carry mineral oil so you can't use it or compressor failure will occur. However Mineral oil is not compatible with PAG oil. In a conversion you must either flush everything out and replace the compressor to remove the mineral oil or use simply use ester oil. The mineral oil will just find a low spot and settle there but as long as you do not add PAG oil it won't hurt anything. This is important because an inexperienced person/shop may add PAG since most of the OEM's use it and then you have problems.
In my opinion the two best choices are buy some overpriced R12 on craigslist or do a legitimate conversion. A legitimate conversion would be something like a Vintage air system with a newer style (high pressure compatible) compressor and proper pressure sensors.
I still do R134a conversions even on my cars because it is easily available but I expect to service the system at least once every summer.
However
I have not tried the R12 replacements like duracool, F12 and such. I tried the Freeze 12 when it was available and was unimpressed.
Anybody have experience with those I would love to hear about it.
I used 414B "hotshot" in my former B van and still use it in my Shadow. The van came with R12, the shadow being a 94 was a ba$tard configuration from Chrysler even when new. The system was designed for R12 but for the final year they switched to 134A and I remember even when new it went back to the dealership several times under warranty claims because the A/C never worked right. My dad had an A/C savvy friend put R12 in it just after the warranty was over when R12 was still mostly readily available at the time.
In 2004 we switched to 414B and it ran just the same as an R12 system. 414B was phased out a couple years ago since its still more damaging to the atmosphere. I have not tried the successor to hotshot yet since I still have several pounds of 414B in stock.
