The present invention is concerned with diaphragm operated valve devices involving the use of a diaphragm piston for alternatively switching delivery of fluid pressure between passageways conducting such fluids under pressure. 
Such diaphragm type valve devices include, for example, a presently known form shown in FIG.  1 and labeled PRIOR ART.  In this valve device, two valve chambers 1 and 2 separated by a diaphragm piston 3, are formed in a housing 4.  Fluid under pressure may be supplied to valve chamber 1 from an input port 5 through a supply passageway 6, and fluid under pressure may be supplied to valve chamber 2 from an input port 7 through a supply passageway 8.  Diaphragm piston 3 comprises a diaphragm 9 peripherally secured between two sections 10 and 11 of housing 4, and a piston follower 12 secured concentrically in the central portion of said diaphragm.  Follower 12 carries valve members 13 and 14, one on each side thereof which, by axial movement of diaphragm 9, can be seated on oppositely facing valve seats 15 and 16 fixed in the respective sections 10 and 11 of housing 4.  Valve seats 15 and 16 are open, respectively, to output ports 17 and 18.  Upon supply of fluid under pressure to valve chambers 1 and 2 from passageways 6 and 8 through input ports 5 and 7, if the pressure of fluid in valve chamber 1, for example, is greater than the pressure of fluid in valve chamber 2, diaphragm piston 3 moves axially rightwardly, as viewed in FIG.  1, due to the pressure difference acting on the diaphragm piston, to seat valve member 14 on valve seat 16.  Consequently, the connection between valve chamber 2 and output port 18 is cut off, and valve chamber 1 and output port 17 are connected.  Fluid under pressure entering into valve chamber 1 with a higher pressure is discharged via output port 17 through a delivery passageway 19.  When the pressure of fluid entering into valve chamber 2 is higher than that of fluid under pressure in valve chamber 1, the process opposite to that described above will occur so that fluid pressure is delivered via output port 18 and delivery passageway 19.  In this way, of the fluids under pressure entering into the valve chambers 1 and 2 from the input ports 5 and 7, the fluid with the higher pressur will be delivered from one or the other of output ports 17 or 18 due to the switching of the diaphragm piston 3 induced by the pressure difference between the respective fluid pressures acting on the opposite sides thereof. 
In such conventional valve devices, when diaphragm piston 3 is in the neutral position, in which it is shown in FIG.  1, valve chamber 1 connected to the input port 5 is connected to output port 17, and valve chamber 2 connected to input port 7 is connected to the output port 18.  Therefore, when the fluid under pressure is fed very slowly into valve chambers 1 or 2 from input port 5 or 7 through the passageway 6 or 8, diaphragm piston 3 may not be moved sufficiently to seat one or the other of valve member 13 or 14 on valve seat 15 or 16, so that fluid under pressure flowing into valve chamber 1 or 2 may flow back to input port 5 or 7 from output port 17 or 18 through passageway 19 or 20, output port 17 or 18, and valve chamber 1 or 2.  Consequently, axial movement of diaphragm piston 3 may be unduly delayed or not be accomplished at all.