A molten metal pump assembly and method to fill complex molds with molten metal, such as aluminum. The pump assembly includes an elongated shaft connecting a motor to an impeller. The impeller is housed within a chamber of a base member such that rotation of the impeller draws molten metal into the chamber at an inlet and forces molten aluminum through an outlet. A first bearing is adapted to support the rotation of the impeller at a first radial edge and a second bearing adapted to support the rotation of the impeller at a second radial edge. A bypass gap is interposed between the second bearing and the second radial edge. Molten metal leaks through the bypass gap at a predetermined rate to manipulate a flow rate and a head pressure of the molten metal such that precise control of the flow rate is achieved.