Paul,

   There a piece missing from this puzzle.  Help me understand.

> If I were returning fuel to one of the main tanks, I would think that venting the sump would be appropriate, but since I am sending fuel undere pressure back into the sump tank, I am concerned that once the sump tank is full, the additional fuel pumped back into the sump tank would take the path of least resistance....out the vent instead of back up the fuel line into the tank?  
>

   When you first power-up your fuel system, you have a
full header tank (or at whatever level remaining in the
mains, if there's overlap).  You have an empty return line
from the regulator to the header tank.  As soon as your
fuel pressure exceeds the regulator setting, fuel begins
pushing air out of the return line, into the header tank,
creating a "head" pressure against the mains.  At some
point, fuel pumped out of the tank equals what's coming
back - until you start the engine.  Now less fuel is
coming back than is being pumped out, and the difference
should be made up from the mains.  

   However ... you have bubble of air and vaporized fuel
in the header tank - only as long as the head pressure of
the mains exceeds the pressure in the header tank, will
fuel flow from the mains to the header tank.  Heating the
fuel will increase the pressure of the bubble.

   I suspect that the header tank should be vented back to
the mains.

   What did I miss?

Dale R.
COZY MkIV #1254