Being that old chances are that it is an R-12 system which R-12 is hard to come by and if you can find it, it's very expensive. I would assume your options are to retro the old system to 134-A or just replace the entire unit. You may be able to get a tax credit depending on the particulars govt. tax credit and efficiency of the replacement unit. I don't believe residential systems have a low pressure cutout so it's basically running itself into the ground and will eventually cut out all together from internal over temp.
