Originally Posted by
cyccommute
That would depend on the kind of valve. Schrader valves have to be held open to get air in the tube. Presta valves are check valves with a low cracking pressure. Air goes in and the valve closes once there is a pressure differential between the air supply and the pressure in the tube. A Presta valve is supposed to act as a valve when air is being added.
While presta valves do close when there is pressure in the tube greater than whatever is on the other side of the valve, this isn't necessary while pumping. The chuck is the only thing difference between presta and Schrader pumps, and the only difference there is that Schrader pumps manually push in the valve core to open it. The pump will still have a check valve that keeps air from flowing back in.