The wide inside bottom part of the extender is too short, meaning that you can't keep the Presta valve fully open, or else it will be pushed down, letting all the air out. If the valve is threaded down a bit, it works, but air will come in more slowly when pumping. See photo: red extender is the normal size, one I already had, the black one is the new one, the difference on the inside seems similar. I had to thread the valve by ~2 mm to keep air from escaping.