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.
I believe that once the the pressure is equalized between the pump and tire, there's nothing keeping the valve either open or closed. Therefore, if you don't have a very good seal between your pump head and the tube valve, you'll watch the pressure slowly decrease as air leaks out.