I agree with GregS. Stop leak products sometimes work and sometimes don't. The goop they leave behind is not at all good for the compressor, and let's face it, if you have an 80s era (or earlier) MB, you don't have the world's greatest cooling system to start with. You don't want to bring on a compressor failure when you may just have a bad o-ring. If you system has some refrigerant in it just top in off with a leak finder (not sealer) product - I prefer the one's with UV dye. Then, look for the leak. AC systems repairs are definitely DIY on most MSs (at least if you don't have go under the dash to replace the evaporator). Lots and lots and lots of information here on repairs, just try a search. Good luck.
82 300 SD
77 450 SL (gone)