Just like vames and others have stated, check the washers. Did you use new washers or reuse the old ones when you installed the new petcock? I just experienced this same issue....at first, I thought it was the rubber dial washer behind the shutoff lever. It leaked like crazy after replacing it. As it turned out, the fiber washers were all shrunk and weren't sealing at all. They sealed fine before I had the petcock off the tank (while it was being painted), but after being off the bike and not soaked with gasoline, they shriveled up to nothing. When these don't seal properly, gasoline seeps down from the tank into the sediment bowl portion of the petcock, thereby bypassing the shutoff part altogether. That's why you are getting the leak most likely.
I couldn't find any fiber washers locally, so I ended up getting Nylon washers from Lowes and it has worked well. Supposedly, Nylon is gasoline resistant from what I've read, so that's what I used. Replace the washers and I bet your leak will stop.
