If there is any where on the sear of the gun you are installing the valve in, the bolt probably sits farther forward. This could cause leaking if there are shims installed and it allows the vent hole to be exposed.
Originally Posted by Crazy685
It probably just needs to be tuned. If the valve was tuned prior to you buying it, then it probably just sits in a different position from the setup it was tuned in. Remove all the shims from the powertube. If it still leaks, then tune it by using a smaller carrier size with the same white carrier oring.