Originally Posted by PistolRogue
Either the valve pin isn't dropping down through the hole of the valve body, or your spring is getting cocked, from what it sounds like anyway. I normally leave the valve spring stuck to the cupseal, drop it in and make sure it passes through the hole, THEN install the pierce block. All of this is done with the body pointing muzzle end up. It'll probably make things easier to remove the bolt, hammer, and drive spring so that you can be sure the hammer isn't interfering with the valve pin coming through the valve body.
I was thinking of the hammer. Oops