Two-port valve for vacuum operation

Disclosed is a two-port valve in which a fluid passage between two ports is gradually openable without a rapid flow rate, by stepwise lifting a valve member off a valve sent using two pistons, to open the valve in a phased manner. A cylinder portion for driving a valve member has a first piston through which a valve shaft slidably extends, a sleeve movably disposed in a second pressure chamber to limit an operating stroke of the first piston , and a second piston slidably disposed in the sleeve and connected to the shaft . In an initial phase of opening of the valve, the pistons acting like a unit are together moved to a position at which the first piston contacts the sleeve , to slightly open the valve member . Thereafter, the valve member is moved by the second piston to its fully open position.