Originally Posted by
FBinNY
BTW- valve orientation while pumping doesn't matter because the force from pressure behind the valve dwarfs the effects of gravity on the 2 gram (if that) moving part.
It might be stated more precisely to say that *air movement* through the valve dwarfs the effects of gravity on the ~2 gram part.
True in most cases, but not always true. If the leakage rate of the presta head seal is moderate (i.e. average, somewhat normal), there is pressure loss the moment that pumping effort ceases, and until the pumphead is perhaps sharply pulled off the valve stem, this lost air's effect on pressure is cumulative and unknown.
With the valve inverted at 12-O-clock, the air flow (however slight) can become strictly one-way, as long as the chuck isn't so sideways as to hold the valve open.
Riders in the old days knew that the valving action when using valve-less frame pumps worked better with the valve up top of the wheel, the better to reduce the incidence of a blown-back pump handle, but it was perhaps even more important to hold the pump's head squarely with the axis of the valve stem.