Could just be a stuck valve if the KP hasn't been used in a while.
Cheap and dirty fix is to introduce some oil into the valve. You can do this by pulling the bolt back and dripping oil into the gas port in the bottom of the barrel. Then repeatedly dry fire the marker. Drop in another 12 gram and pressurize the marker. Dry fire it some more to see if you can get the leaking stopped. It may take a couple of tries. This MIGHT seal it up. If not, the valve core is probably shot and will need to be replaced. If you already have the tool this is really no problem. Just take care to remember the order in which everything came out and put it back together thusly. BTW you don't need the lead gasket. Pitch it out. It is just the old fashioned way of preventing the valve screw from backing out over time. You will wear out the valve core long before that happens anyway.