My Petcock was leaking fuel when you shut it off. It is thirty years old. I took it apart and found that there is a round washer with three holes that was all cracked from age.
I used a piece of neoprene rubber 1/16" thick. I pressed it on the opening of the petcock body to get an impression of the circle and cut this out. I let it soak in gas overnight and the gas didn't seem to hurt it.
To make the three small holes I found a nail that would just fit into one of the small holes. I filed the end of the nail flat with nice sharp edge. I then placed the neoprene on a piece of wood and knocked the holes out. Put the neoprene in the petcock and it works great.
A new petcock is about $18 - $24. The body and all the other parts were in good shape. This is a .50 cent fix for those of you with a leaking petcock.