I have a 91 300SEL W126 chassis. It has R12 freon which is running low. The compressor works fine, and i think it is just a slow bleed from somewhere b/c freon has not been filled in at least 7 years. I do not want to change out the compressor, etc. Any pearls on the R12-R134 conversion kits available at AutoZone, or the hydrocarbon based kits available on-line???