R12 has been banned from sale for many years so you won't find any (legally) anywhere. R134a has similar properties to R12 and has been used in the vast majority of car air con for the past 8/9 years.
R134a cannot be used in a system that originally used R12 unless the system is specifically converted. The main problem is the different type of compressor oil that is used. Most R12 systems use(d) a mineral refrigerant oil whereas a R134a system MUST use a synthetic "ester" oil. If an empty R12 system is topped-up with R134a, the R134 reacts with the original mineral oil causing tacky oil globules which are likely to block the evaporator expansion valve or fine capillary tube, whichever is fitted. R134a is also not capatible with some types of rubber that were traditionally used for hoses and seals in R12 systems and results in many leaks.
Converting an R12 system to use R134a requires all old compressor oil to be drained and the system fully flushed. A new filter/drier should be fitted and It is advisable to change all hoses unless the system manufacturer can specifically confirm that the original hoses are R134a compatible. It is also advisable to change all seals and gaskets as R134a has a much smaller molecular structure than R12 and a fully gas-tight R12 system could leak like a sieve with R134a. Also, once any air con system has been opened, once re-sealed, a deep vacuum needs to be pulled to withdraw all air and moisture before oil and/or refrigerant is added. Failure to pull a deep vacuum will result in the air in the system throwing out all the system pressures so that the system will be ineffective and the moisture in the air reacts with the refrigerant to form an acid which will lead to all sorts of other problems!
HOWEVER, because of the problems and cost of converting from R12 to R134a, various refrigerant manufacturers produce a R12 "drop-in" replacement, the commonest (in my industry - rail vehicle air con) being R401a. British Oxygen (BOC) distribute DuPont refrigerants who have a R401a gas which goes by their (DuPont) product name of MP39. BOC have depots all over the country and all have trade counters where you can buy R401a/MP39 although I don't know if it is available in small aerosol size containers. Alternatively, look in the yellow pages for refrigeration part distributors (HRP, Dean & Woods, Refrigeration Spares Ltd, Kooltech.... there are loads around) and give them a ring and ask for a R12 drop-in replacement, they all have trade counters where you can purchase over the counter
For your information, it is illegal to release any form of refrigerant into the atmosphere and there are EU directives in the pipeline that will prevent anyone from handling refrigerants unless they hold a refrigerant handling certificate. This is probably why Halfords are stopping selling refrigerants.
Although R12 drop-in replacement gases are more expensive than R134a, the amount used in a car air con system is so small that it shouldn't make a great difference. The whole point of the R12 drop-in replacement gases is that, in theory, you don't need to replace O rings and seals although in practice it is advisable. Most air con engineers that have used any of the drop-ins will have their own experiences and will make recommendations accordingly.
It should be noted that all refrigerants have a direct pressure to temperature correlation meaning that within a sealed environment such as an air con system, if the pressure of the system is measured, the temperature of the refrigerant will also be known. As the pressure rises, the temperature of the refrigerant rises. R134a and all the various R12 drop-in replacements that I know of all have a slightly higher pressure to temperture correlation than R12 at higher temperatures. I haven't got any pressure/temperature tables to hand but as a simple example, if R12 has a pressure of 200 psi at 100 degrees C, R134a and the drop-ins have a higher pressure of 220 psi at 100 deg. C. This is not that important until you get very hot days when you might find that your high pressure cut-out switch trips. Systems designed for R134a will obviously take this higher pressure into account. However, R12 systems that are converted to R134a or R12 systems running with one of the drop-in replacements need to have minor tweeks to cope with the higher pressure. The safest method is to reduce the refrigerant charge by 10-15%. Within my part of the air con industry we usually reduce the refrigerant charge by 10% and increase the trip pressure of the high pressure switch by 10% I suspect that the high pressure switch on Eunos/MX5 air con is non-adjustable (and unless you know what you're doing, please don't attempt to adjust the high pressure switch)
I don't recommend that anyone should attempt any top-up or adjustments themselves but this information might enable you to ask certain questions to determine if the man about to repair your air con really knows what he is doing! Car air con is big business these days and there are plenty of cowboys out there so be careful
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