To revert back to R-12 you need to thoroughly flush out the oil in the system and reload the system with R-12 and the proper oil for R-12. The oils used in R-12 and R-134a are not compatible with each other's refrigerants.

Don't flush the compressor with flush - instead run a lot of mineral oil through it, as mineral oil is the R-12 lubricant. The rest of the system you can flush with flush solution from a flush gun or with a compressed air supply.

My suggestion is that if you determine the compressor used in the R-134a retrofit is a "Four Seasons" compressor - just go ahead and replace the compressor now during the rest of your work. You'll get a better compressor and won't have to clean out the R-134a oil (PAG Oil I think) from it.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles