I don't think you quite understand the fuel system on the B757. There aren't separate forward and aft tanks -- the switches on the overhead panel are for forward and aft fuel pumps in each wing tank.

Simply put, there are 3 fuel tanks -- left wing main, right wing main, and center. Each tank has two fuel pumps. In the main tanks, the 2 pumps are called "forward" and "aft". In the center tank, the pumps are called "left" and "right". [Although one pump can supply sufficient fuel, there are two of them in case one goes bad.]

Before the flight, fuel is added to the center tank only if you will need more than the two main wing tanks can hold, which is approx. 30,900 lbs/14,045 kg combined. During flight, the fuel in the center tank is burned off first. When it is gone, then the fuel in the main wing tanks is used.

There is a good discussion of the B757's fuel system here. Actually, the whole website there is interesting. Hope this helps!

Stu