> the work of a pump is independent of the depth of the fluid that
> it's working in
.
Sort of true, if interpreted correctly. All other factors being
equal, the work is dependent on the height difference between the
surface of the fluid and the point to which the fluid is being
pumped. It doesn't matter how far below the surface of the fluid
the pump is located, because the fluid's weight "helps" pump the
fluid upwards towards the surface. Keep in mind that the height
of the fuel injection hardware is fixed in a car. So, as the fuel
level drops, the height difference between the fluid surface and
destination increases, and the pump(s) has(have) to do more work.
.
Hope this made things more clear.
.
-b.