A method and apparatus for improving the efficiency of a drill string that includes drilling pipe, a dump valve, a positive displacement motor and a rotary drill bit driven by the motor utilize a dump valve that does not depend upon a pressure loss to maintain the dump valve in a position which closes communication between the drilling string and the annulus between the drilling string and the well bore or casing. The dump valve includes a flow sleeve that is axially moved to close outlet passages to the annulus by the flow of fluid through the outlet passages rather than a pressure drop across the flow sleeve.