There's no valve in many or most presta-only pumps.
The valve is the presta valve itself, which must be in the 12-0'-clock position on the wheel to reliably act as a one-way valve, due to gravity.
One can expect the valve to mostly work in other positions, as the valve can also be blown shut, but the core can and will hang open occasionally especially if the pump head is tilted on the valve or the pumping action is too slow. It could even be dangerous if the handle slipped out of one's hand, might nail you in the crotch, for example.
David Snyder (don't) ask me how I know in Auburn, CA usa
Paul Andrews wrote:
> Hi Eric,
>
> Is the one-way valve for a Silca frame pump in the head? I have never
> disassembled mine, but I did notice an unusual occurance the last time I
> used it: the pump handle did not stay in, but rather came right back out
> again under pressure. This happened a couple of times, leading me to
> believe that a flap valve was on its way out.
>
> If anyone has a spare Campy metal head, I could use one - mine is quite
> rusted
>
> > Ft Lauderdale, FL, USA