Originally Posted by mercator

I once lent my pump to a fellow with a similar hangup as the OP. To whit, when he released the chuck and heard the air escaping at 110 psi he was convinced the pressure in the tire had dropped and insisted on pumping it up again. After he did it four times, I explained that none of the air was coming from the tube, since the presta valve closes instantly, it was just the air in the hose.

I'm not sure if I convinced him, he seemed to have a high degree of confidence in his mental model, but I took my pump back and he rode off. Maybe it was [MENTION=491785]mjac[/MENTION]?