Originally Posted by
big john
It doesn't what, read the pressure in the tire? He said the valve closed as soon as you stopped pumping, therefore you couldn't get a reading until you applied pressure to open the valve.
As to your other point, I don't know how much difference there would be, maybe very little but I would have to assume at least a little higher in the tire/tube to force the valve closed. This is assuming you subscribe to the theory that the pump head does not depress the valve. If you've ever heard a lot of air escaping as you have a tough time removing the pump head then you know the valve is being depressed, at least in that instance.
As with the original question, all of this is actually moot.
Wrong. As someone else (
DaveSSS ) posted earlier as soon as you stop pumping the pressure in the tube forces the valve closed. You don't lose any pressure from a presta valve when removing the pump head. The air you hear is coming from the pump, not the tube. It's no 'theory'...the pump head doesn't depress the valve, air pressure from the pump does. You continue to get a pressure reading when you stop pumping because there is still positive pressure in the pump/hose. Oddly enough it's the same as what the pressure is in the tube.