Thanks Harry, Ive always wondered about that.

So any perpetual-motion machine using permanent magnets
as a propulsion system would only ever be 'virtually' perpetual...


Now back to topic...
But what about if we re-design Ventilators doo-dad a bit...
First, throw away the valves altogether.
Then we fill the air side with another fluid.
One that CANT mix with water, for-instance alcohol.
Then find a new ball with a density that allows it to
still float in water, bit sinks in the alcohol. (or vise-versa)
Then we extend the vertical tubes until the ball gains
enough momentum during both the rise and fall times to be
able to "pierce" the surface tension between the two fluids...

That way, we no longer worry about the valve energy requirements,
and if any fluid gets 'dragged' into the wrong tube it will
then either rise or fall as a bubble (like a lava-lamp),
and re-merge with its own type.

[EDIT]After further thought, I think we still need the top valve, just to keep
the 'up' tube fluids 'mostly' separate from the d'down' tube.
But it no longer needs to be perfect, as long as we get the
ball moving fast enought to push it open, its allowed to drag
a few drops through with it...