a pneumatic controller for actuating a retarder of the motion of a vehicle . inlet and exhaust valves are actuated by a pair of diaphragms responsive to the pressure of the air applied to the brakes of a vehicle and the pressure of the air actuating the retarder . the valves are preferably on a common stem and operatively associated with the diaphragms by an actuator rod responsive to the resultant of forces produced by both of the diaphragms .