Originally Posted by cyccommute

The reason that chucks have either a different hole or a different configuration is because of the nature of the valve. Yes, the pump has it's own check valve but that check valve allows for a small amount of pressure change in the hose. Since the Schrader valve is held open, that doesn't matter for that valve. But for a Presta valve, the change causes the valve to close. Pressure introduced when the piston in the pump is pushed down causes it to open again. It goes through a cycle of opening and closing.



I'm not saying that the pump doesn't have a check valve. I'm saying that the Presta valve closes at a very low pressure differential. Most all hoses leak some air around the valve stem. This can be because valve stems have threads or because of a poor fit but nearly all of them leak some air. For a Schrader valve, that leak is coming out of the tube. For a Presta, the valve closes until the pressure outside the tire is higher (by a very small amount) than inside.

The chucks have to be different because of the way that the valve has to be used.