A pneumatic pressure regulator assembly (10) including a regulator housing (12) having an inlet (50) adapted for fluid communication with a supply of pneumatic pressure, an outlet (52) adapted to provide pneumatic pressure to a downstream pneumatically actuated device at a regulated pressure, and a regulator valve assembly (46). The regulator valve assembly (46) has a valve member (58) movably supported within the regulator housing (12) between an open and a closed position. A piston assembly (48) acts to bias the valve member (58) to its open position when the outlet pressure is below a predetermined value. The piston assembly (48) includes a pressure responsive surface (106) defining a geometric shape with a major and a minor axis wherein the major axis A is greater than the minor axis B. The piston assembly (48) is responsive to pneumatic pressure flowing between the inlet (50) and the outlet (52) to reduce the biasing force acting on the valve member (58) such that the valve member (58) moves to its closed position when the outlet pressure exceeds a predetermined value.