DON'T, Allow your vehicle to be converted to a blend PERIOD. Blends are a mixture of refrigerants and considered a contaminated system by most reputable air conditioning shops. Farrell's A/C Service doesn't use blends or service a system containing a blend.
DON'T, Allow your older R-12 system to be converted to R-134a until you've been told all the facts and disadvantages of R-134a. Disadvantages like, R-134a doesn't cool quite as well as R-12 (takes longer to cool your vehicles interior), leaks out faster (Smaller molecules), much harder to find a leak even with today's best leak detectors (That's why the manufactures now install dye in most new vehicles), runs at higher pressures (Higher pressures are the leading cause of compressor failure), and will shorten the life of you air conditioning system.
The biggest DON'T of all. DON'T allow anyone to install a sealant into you air conditioners sealed system. Even if it does temporarily stop the leak, it's not a permanent fix, and the damage caused by this sealant can and will destroy the entire system and make the system unfixable. True professional air conditioning shops wouldn't consider putting that junk in there vehicles, why let them inject it into yours.
DO, Call Farrell's Auto A/C Service. Trust me, you won't regret it.
The views expressed here are strictly the views of Glenn Farrell, owner of Farrell's Auto Air Conditioning Service. With over 37 years experience in the air conditioning and refrigeration trade. Conversions of R-12 systems to another refrigerant is mandatory in Canada and some other countries. R-12 is still allowed in vehicles in the United States according to the latest EPA reports. http://www.epa.gov/ozone/title6/609/index.html