An instrument includes an instrument body having an opening at one end. A valve is mounted within the instrument body, the valve having an opening at one end. A lid is integrally molded with the valve and hingedly movable with respect to the valve opening. An actuating arm is operatively connected to the lid, the actuating arm transmitting forces from an instrument movement mechanism to the valve door under both tension and compression. As a result, the lid is opened and closed substantially solely by the actuating arm.