Originally Posted by
Vince Canepa
If the idea of the Presta valve is to prevent any air loss when you insert the valve (more important with an "onboard" pump than a floor pump) then tapping the valve stem before inserting the chuck defeats that purpose, doesn't it?
The idea isn't to prevent any air loss when you insert the valve in the pump head since you are going to add additional pressure anyway. Tapping the valve stem breaks the seal so air can enter the tube at a reasonable pressure differential and you will make up that small loss with the pump. The real intent is to prevent air loss when you
remove the pump head.