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Todd:

You should ALWAYS return to the tank you are supplying from.
try this scenario.
Both tanks full on takeoff.
Pump from left and return to right.
Fly low and dust someones crop with 100 LL when you force gas out the right
tank vent.

Hank
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Well, that would be a stupid thing to do.  Yes when both tanks are full you
need to return to the same tank you are using.  Most pilots could be trusted
to do that at least as much as trusting the pilot to select the full tank
when one is empty.  Ususally works fine.  Right Ed?  ;-)

Dave Leonard