: Erez, thanks for the links incase the plungers don't work.

: Mick, it would seem to me that you wouldn't need the teeter or springs since
: the rubber plunger acts as a spring. Let say you press on the plunger
: rubber with your foot. The air pressure closes the oneway valve on the
: bilge line laying in the water and exhaust the air out the oneway valve on
: the discharge line. Then when you lift you foot off the plunger it springs
: back out filling with water as the oneway builge valve opens.

My guess is that the plunger is does not have enough restoring force (or is certainly not quick enough) to lift the water up into the diaphram when the yak's internal water level is below the pump. So a spring or teeter is probably required.

:A vacuum
: must be created in the discharge line at this time in order to close the
: dischage valve and I can't really picture that.

For simplicity and odd orientations, it's probably best that the default position for the valves is closed anyway. Like the ones in your respirator.(Like Erez said, the respirator valves are too weak and would collapse from the internal water pressure, but the idea is good - use a thicker gasket and a stronger return rubber nub.)

-mick