Originally Posted by
2 wheeler
Hmmm... I've been using presta valves for about 38 years and can't ever recall any pump head or adapter that "unseats the presta valve stem automatically". I always have to depress the valve to break it's seal before pumping up a tire. Any depressing of the valve stem, that you mention, would have to be automatic and momentary, or air wouldn't stay in the tube.
Could you enlighten me?
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.