SyEng, inlets for the center tank pumps are not located at the lowest point of the tank. In fact, there is a substantial amount of residual fuel which they can't touch. Once this level is reached, the center pumps are incapable of producing pressure.
Fuel feed then commences from main tanks, while automatic scavenge pumps slowly and automatically transfer the residual center tank fuel to the main wing tanks. Typically, the center tank is literally empty after a flight of this duration.