It would be better to go ahead and convert it over to R-134a. The kits are cheaper than the R-12.
You might also try mixing half propane and half butane and putting it in the system. It takes roughly 10 ounces of that mixture to equal 16 oz of the R-12. This is basically the same mixture that they sell called OZ-12a or freeze-12. Make sure you have plenty of oil in the compressor.
[rant]
Some wacko with no idea how freon 12 work seems to think that it can get into the atmosphere way up above all of the clouds and deplete the ozone, you know. Have you ever thought that the lightening is replenishing all of that ozone? and what about all these ozone alert days? haven't we had a lot more of them since we banned that nasty R-12? And how about the law of physics that states that matter can neither be added or taken away? Isn't it true that the patent was just about to expire for R-12 when the government all of the sudden got wind that this stuff was bad for the environment. [/rant]