Retrofitting an air conditioning or refrigeration system from R-12 to R-134a requires replacing the mineral oil with a polyolester lubricant, replacing the filter drier, and in some cases, adding a flushing agent. You can read our content for detailed information about how do you convert r12 to r134a.
Vintage Air offers the professional A/C mechanic a complete line of R12 to R134a conversion fittings, hoses, and adapters.
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https://www.vintageair.com/r12-to-r134a-conversion/