In a valve assembly (10) exhibiting zero dead volume the port head (12, 112) is mounted on a solenoid (14) and defines an inlet port (16, 116) on one side of the valve seat (20, 120), and two outlet ports (18, 19, 118, 119) on the opposite side of the valve seat. A flexible diaphragm (22) is seated across the valve seat (20, 120) between the port head (12, 112) and the solenoid (14), and is sealed along its peripheral edge. A spring-biased valve rod (24) is coupled to the opposite side of the diaphragm (22) relative to the valve seat (20, 120), and the solenoid (14) is actuated to move the valve rod (24) and diaphragm (22) between open and closed positions. The valve assembly (10) is opened by actuating the solenoid (14) to drive the valve rod (24) and diaphragm (22) away from the valve seat (20, 120) and permit the passage of fluid across the valve seat between the inlet (16, 116) and outlet ports (18, 19, 118, 119).