Posted for Mark Steitle <msteitle@mail.utexas.edu>:

David,
I guess it comes down to how soon the system will go into melt down.  I see
it like a water cooler... why do they put the spigot on the bottom instead of
the top.  You'll get more water out of the cooler if you draw off the bottom.  So,
in your cooling system, by drawing off the top of the radiator, if a leak were to develop
you would run out of water sooner than if you were drawing off the bottom.  From
my brief experience with the Mazda rotary water pump, it doesn't appear to take
much air in the system to shut the pump down.  I would like my system to be able to
continue circulating water for as long as possible, to enable me to get back on the ground.

I remember when auto manufacturers switched to cross-flow radiators.  It was
normal for the coolant level in those systems to be about 6-8" below the top
of the tank.  If they tried to make the top hose the outlet, they would suck air
and overheat/ruin the engine.  I don't know if your system would have this
same characteristic or not.  But, if you were to get a significant amount of air
into the radiator, it would have the same result.

Or maybe I don't fully understand your design?

Mark S.