Originally Posted by rpenmanparker

Sure, my "Hurricane" brand pump head actually depresses the valve stem when you lock it on, so freeing up the valve stem beforehand isn't necessary. As Fiery said, you have to lock the head on quickly to not lose much air. If I were using a Schrader valve with an adapter, it is unlikely that this would take place. That could be why you have never seen it. But as I said before, just quickly punching the valve stem inward very quickly sets it free. After that the pump pressure at the bottom of the stroke is enough to keep open it up on each stroke.