from my little experience working with AC systems, AC leaks very easily beacuse the molecules that make up refrigerant like R-12 and R-134A are very small… and the teeniest tiniest holes in lines or warn out seals can result in leaks…
what i have experienced with this is lots of fittings have O rings and they get old and if they get tampered with they leak and well you simply have too replace the seal weather it be a O ring or what have you and tighten too proper specifications in order too fix the issue… before you do that you should properly drain or i think the better turm is evacuate the AC system first, then once you have installed the new seals or what ever you must do and have the system evacuated again too get all the moister you can out of the system, and have the system recharged with the correct amount of specified refrigerant…