If you push the inflator of the pump on too far on a presta valve the valve end will be pushed down (open) and you may experience the air pushing back out of the tube. Try not pushing the pump end on so far, or push all the way down and then pull back a bit. On a most pumps there is a check valve that could be bad.
http://www.bikepro.com/products/pumps/silcfloor.shtml
Last edited by cny-bikeman; 07-26-07 at 03:02 PM.