Bike maintenance is something we take seriously here at Art's Cyclery. We want to make sure your bike functions perfectly every time you go out for a ride. Whether you're riding mountain or road, we'll be there every step of the way to make sure you have the best cycling experience possible.
Air doesn't escape from your tire at all. There's a common misunderstanding of where the lost air is coming from when you remove the pump head from the valve, as it's likely all from the pump hose itself.
With a Presta valve, the air pressure inside the tube is what keeps the valve shut. When the pressure inside the pump is greater than the pressure inside the tube, the valve opens and air flows into the tube until the pressure equalizes. This design ensures that there are no moving parts inside the pump head required to push the valve open when pressurizing the inner tube that could then hold the valve open during the fraction of a second that the pump head seal is broken and the valve core is depressed.
Us mechanics here at Art's Cyclery prefer to use Presta specific pump heads because they do not have the internal mechanical component that holds the valve open, unlike most dual purpose pump heads that are designed to work for the Presta and Schrader Valves.