I would bet that you have aluminum adapter ports. You will have to recover the refrigerant to repair this. I would strongly suggest that you buy steel or brass adapter ports and replace both adapters while the system is empty. With these high quality adapters, you totally remove the original shraeder valves.

You can get these at www.acsource.com or your local automotive a/c supply house. About all you can get at the auto supply stores are the junky aluminum adapters.

FYI, there is a special tool for changing shraeder valves in an original R12 system without losing the charge.

Good luck,