Hi, I have a vw beetle & my ac valve does not work.?

I would like to add some freon to my car but i was told that the valve inside the ac compressor doesn't work. Then the option that I was given was that is if I wanted some freon instead of replacing the ac compressor I could put a sealant to the valve I guess?? prevented from leaking freon. BUT HOW LONG WILL THE FREON LAST IF I DO PUT A SEALANT ??? PLEASE HELP!!!!