How a Presta valve stays shut
Inside a Presta valve, a small movable stem sits against an opening in the valve body. Air pressure inside the tire pushes the stem toward that seat, helping keep air from escaping. The little knurled nut at the tip screws down to hold the valve closed; before inflating, you unscrew it so the stem can move.
That nut is not a cap you remove. Loosen it a few turns, then press the tip briefly: air should hiss out, confirming the valve is open. The tire’s pressure still presses the stem against its seat, but the pump can overcome that pressure.
The pump has to push harder
A pump first seals around the valve, then forces air into it. For air to flow into the tire, pressure at the pump must briefly be higher than pressure inside the tire. That difference nudges the valve open and drives air inward. When pumping stops, the tire’s higher internal pressure pushes the stem back toward its seat, limiting air loss; tightening the small nut secures the closure.
This is why a pump that is not firmly attached can seem to do nothing: it may not be sealing well enough to build the pressure needed to open the valve. And if the tip nut is still screwed down, the valve cannot move freely, so the pump is pushing against a closed entrance.
