Had my '90 Celica with an R12 a/c system regassed on friday. After 4 years if was not getting as cool as it had in the past. The interesting point is that it was regassed with R134a, without modificatuion.
I'd rung around during the week and been referred to a local garage that could regas with R24, a drop in replacement for R12. Unfortunately they'd run out of the gas, and were having trouble getting any more, so it was back on the phone again to another Co that advertised regassing R12 systems.
Had quite a chat with the guy, and when I asked if he was using R24, he said no, he refilled them with R134a. I expressed surprise, and told him that as far as I knew R134a could not be used in R12 systems. Prone to leaks etc. He replied that that was what he thaught until he spoke to a chemical engineer at a trade a/c meeting, when he was told that the real problem was that the seals in an R12 system could not take the higher pressures of a R134a system, but if less R134a was used, the pressures would be lower, and within an R12 systems limits, so it wouldn't leak. At the same time mineral oil should be added rather than the sythetic oil required by a true R134a system. He went on to say that he'd been regassing R12 systems in this way for months, and so far had many satisfied customers.
Of course he could be giving me a load of old bull, but he seemed genuine enough, and so far the a/c is working well. The true test I suppose is if it's still working as well after year or more.
One point in it's favour if it does work as well as he states, is that R134a is a cheaper gas than R24. I paid £60 plus VAT, for a mobile service at my workplace, and it was a complete job. The system was purged for near half an hour, before it was refilled with gas, oil, and dye. A Co some distance away gave me a quote for an R24 regas of £115 inc, and I had to take the car to his premises. Mike