I can't find a good diagram, but here's how it works. The needle & seat are in the lid of the carb, above the bowl. When the float drops, the valve opens & dribbles fuel through the air above the bowl & then into the bowl.

Even if the needle valve was wide open, the only thing that can be sucked back through the fuel line is air.

I cannot explain how changing a fuel pump "cured" a dry bowl problem, but I am certain there is no way to suck fuel out of a carb via the fuel supply line.

Dave