Schwalbe Presta Valves have removal cores, make sure these suckers are tight. Three times in as many years I've found a wheel sitting on it's rim with a flat tire. Last weekend when it happened I went through the bother of replacing the tube so I could ride without worry. When I returned from the ride I tested the tube in a bowl of water and watched one little bubble at a time slowly trickle out of the stem. I tighten the core and put the tube back in my vast inventory of tubes.