Your petcock is shot, save your time, and money and get a new one, the cost of rebuilding it isn't much cheaper, due to you need two separate kits, as they for some reason don't make just one.
New one cocost about $65.00
http://www.partzilla.com/parts/detai...1023-1139.html
Also be sure to order the orings and new bolts, trust me you'll be better off in the long run.
Rebuilding kits are about $60 total.
K&L Supply Diaphragm Only for Fuel Petcock Repair Kit 18-6638
https://www.amazon.com/dp/B0055DWO60..._s1CGub1W9N3PH
K&L Supply Fuel Petcock Repair Kit 18-2723
https://www.amazon.com/dp/B001KO2JRW..._U1CGub05DT5NT
IMHO not worth it rebuilding, also be sure to check vacuum and fuel lines for wear, holes, and cracks, if in doubt replace them.