You can't get freon anymore over the counter. I believe the only way you can get it now is if you have at least a spec. 6 (automotive) A/C certification. If you need to get the compressor running, there's a low pressure switch that will shut the compressor down if the freon load is to low. This prevents the compressor from cycling and buring itself up. However, it is an extremely bad idea to run the compressor without the gas, for this same reason.
The R-12 needs to be done by a facility. WalMart has a conversion kit for about $30 but I do not know if you will need additional cans of charge for the complete system. By '92 need to have the R-12 charge and rumor has it that runs $100. There is a place by I-25 and DTC that came recommended for the job but I can't recall the name now...
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