Here is thread from the old tech board by Slowroad:
http://www.valkyrieforum.com/forum/tech_archive.cgi?read=1080571"I had the same problem when I got my Valk. I would bet that the o-rings around the ports inside the valve are shot. Thats what mine turned out to be. I drilled out the rivets, replaced the o-rings with parts from the hardware store and used small machine screws and nuts to put it back together. been working fine since."
I didn't have go that far on mine. Just used a large drill bit and reamed the metal seat a bit so the ball sits in the hole a bit deeper sealing against the o-rings.
I'm currently adapting my tank to go without a petcock all together, installing an electric fuel valve.
If you follow the thread down he explains more than the above quote does.