Originally Posted by
cxwrench
No...as SOON as you stop pushing down on the pump handle the valve closes. You go from more air in the pump chamber to more air (pressure) in the tube/tire instantly. NO air pressure is lost when you remove the pump head from a Presta valve.
Unless you're using sealant. The sealant can collect on the tube side of the presta valve preventing it from totally closing when you remove the pump head resulting in a slow air leak. When I close the nut on the presta valve the air leak stops. If it bothers me enough I have to remove the valve and clean it.