check valves on air pumps?

[urgle the danish cow]

i discovered that my local store carries many check valves and i am inspired to make an air pump for the ones i buy always break.
i understand the way the pump plunger works but how does the check valve open under higher pressures? does the plunger have to poke it open or does it have to get extremely close?

[Gun Freak]

The air pressure generated on the stroke opens it; once the pressure equalizes, the spring closes it.