Originally Posted by
cyccommute
Yes, a Presta valve opens when the pressure inside the pump is higher than the pressure inside the tire. But as soon as that pressure changes...and, assuming a manual floor pump, it does when the pump handle is pulled up...the pressure inside the tube is higher than the pressure in the hose and the valve closes and traps the air in the tire.
A Schrader valve has to be held open for air to flow into it. You could overpressure the valve and get it to open but you need something on the order of 25 psi higher than what is in the tube to do so. You have to overcome the spring on the valve which is keeping the valve closed.
And the differences aren't just the chuck. The chuck is different because the valves are different. You could hold a Presta open like a Schrader but that defeats the purpose of the check valve.
Generally the difference is just the chuck - otherwise how could pumps come with a chuck that works with or can be adapted to work with both valve types? The little switch on the chuck of my cheap pump does not effect the way the pump functions at the other end of the hose, it just changes which hole in the chuck is connected to the hose.
And while you are correct in the way presta valves work and keep air from escaping, this function is not necessary during pumping as
all reciprocating pumps have a built in check valve - if a pump didn'n have a check valve then it would simply be called a 'piston' and it wouldn't be able to push air into a tube and have it stay there.