The fact that it has ANY pressure in it at all tells you it's sound. If it had a leak any bigger than what will get past the crank seal naturally over many years, it would have had zero pressure.
If it's still running R12, see if you can find some and just charge it back up. I would avoid those R12 drop in replacements if I could as my understanding is they are probably propane or similar.
If it has been previously converted to R134 then off to Walmart for a few cans.
