I would take apart the petcock. If the diaphram is obviously messed up, then that might be your problem. My bike was leaking fuel intermittently, but as the miles wore on, more often then not. It would leak when running, but not when shut down. Really baffling me. THe whole petcock was wet, and fuel was dripping down the back of the tank. I took it apart, and found a really messed up diaphram. I'm guessing it was put in wrong at one time.
Still the petcock was wet, fuel was dripping off of it. But the funny thing was, the tank wouldln't leak a drop off the bike, but only when on the bike. I got in close and finally realized that fuel could be seen leaking out in front of the petcock. Turns out, the left side mounting bracket had been cracked before, patched by some previous owner, but it had cracked again.
Moral of the story is that a cracked tank can appear to be a leaky petcock, and cause you a lot of wonder and grief as you tear into things trying to fix them.