Originally Posted by Stubacca
With schraeder valves it can be a bit of air escaping from the tyre. With presta valves, it's the air pressure in the tube. Presta valves are only open during the actual pump stroke.
Er, what the hell?
1) You seem to be using tube and tyre interchangeably, which makes no sense. You only put air in the tube.
2) Your first two sentences seem to be saying the same thing: when you pull the pump head off the valve, the hiss is air leaving the tube.
3) Your first two sentences are therefore not correct; a lot of the hiss is air leaving the pump's hose and piston.
4) Presta valves are open whenever their plungers are depressed, which occurs whenever a pump head is properly fitted to them.
- Warren