You might want to consider converting to the newer (and far cheaper) refrigerant, I think it's 134A. I recently had a good, local radiator and A/C shop do the conversion, they charged $135 for everything. I can't tell the difference in cooling so it seems to work just fine. The R12 is just going to continue to be a problem so I'd just get rid of it and save money in the longer run.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold