You don't believe me? You put the pump on a Presta valve and don't get a pressure reading, right? Because the pump head doesn't open the valve. On a Schrader valve the pump head opens the valve and you get a pressure reading as soon as you put the head on the vavle. When you pump up a presta valve the air from the pump pushes the valve open, air goes in. When you reach the bottom of the stroke/stop pumping the pressure in the tire is greater than in the pump and the valve closes under that greater pressure. Thus no air is lost.