The main culprit is the incredibly long rubber hoses in our systems. To convert from R-12 to R-134a the right way, first, convert from the York piston type compressor to a rotary type pump. Second, add a conversion kit available to your system including the R-134a type fill fitting. Third, trash all the old hoses and the anti-icing thermostat under the dash, and the old dryer unit in back. Add a R-134a compatible valve & dryer unit, and have barrier-style hoses built as duplicates of your OEM hoses. Third, vacuum the rebuilt system for 30 minutes and fill 'er up, using at least 2 cans of R-134a oil & freon. You can skip steps 1 & 2 if you don't mind refilling the system more often than you used to. R-134a is a physically smaller molecule and will leak easier than R-12 thru old hoses. Be sure to drain & clean all the compressor oil from your old York which has a sump. The old oil will chemically react with the new R-134a compressor oil. Rotary compressors do not have sumps, so vacuuming is all thats needed with such.
