If your system is only low on R12 and you haven't opened it up yet do what my brother and i did. Go on E-bay and look up Freeze 12 kits, they sell for around $40-50. We did this to my brothers 89 Acura Legend and the kit includes the charge hose, one oil charge, three cans freeze 12 and two adapter valves. You will need to remove the schrader valve, put on the freeze 12 adapter vavles. Then put the oil in first after that put two cans only of the Freeze 12. It took about two years before i refilled it with the other extra freeze 12. Be careful of overfilling it, it's like the new freon 134A you put too much and it will actually get warmer.
http://cgi.ebay.com/ebaymotors/Freeze-12-Pro-Quick-Charge-Kit-R12-R134a-Alternate-A-C_W0QQcmdZViewItemQQcategoryZ46094QQitemZ8060926270QQrdZ1