A valve stem-based air maintenance tire assembly and method of operation is provided employing a tire mounted air pumping system, The assembly includes a rim-mounted pressure regulator positioned within a tire cavity opposite an inward end of a tire valve stem to selectively open and close pressurized air flow from a valve stem internal passageway into the tire cavity. An elongate valve stem shaft is mounted within the valve stem air passageway and reciprocally moves axially to close off and open the valve stem air passageway to create or close a pressurized air path into the tire cavity as needed.