Low Wing Aircraft – Lack of “Both” on the Fuel Selector Valve

Hey guys,

I'm having trouble understanding why low wing GA airplanes typically don't have a "Both" on the fuel selector valve whereas high wingers do. At this point my understanding is this:

-Suppose a low wing plane did have a "Both" option. Since fuel is pumped into the carburetor via an electric or engine driven pump (no gravity feed), running one tank dry would cause the pump to just force air from that tank into the fuel system if "Both" were selected, rather than fuel from the good tank. In a high wing airplane, gravity would allow fuel from the good tank to continue falling into into the fuel system. 

That's the basic extent of my interpretation. I'm not entirely sure why, if the fuel selector were set to Both on a low winger, the pump would draw air from the dry tank at the expense of fuel from the good tank. Wise CFIs...let me know what I'm missing!

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