The valve has a casing with inlet and outlet ports, and a valve opening bordered by a valve seat communicating the ports. A flexible diaphragm defines a control chamber and a pressure chamber on opposite sides thereof and moves between an open position clear of the seat, and a closed position against the seat. The valve seat is within the pressure chamber, and the diaphragm is normally urged closed. A flexible bellows extends between and is sealed to the diaphragm and the casing on a side of the diaphragm remote from the seat. The bellows provides a bellows chamber isolated from the control chamber to enclose an effective area of the diaphragm generally equal to the valve opening of the valve seat. The diaphragm has an orifice providing limited communication between the control and pressure chambers and having a size sufficient to reduce fluid flow between the chambers to reduce diaphragm speed and thus chances of hydraulic hammer. The casing has by-pass channels communicating one of the said chambers with the outlet port when the diaphragm is held against the valve seat, optionally the bypass channels can be valved. Another embodiment of the valve for gas applications eliminates the diaphragm, but the bellows similarly isolates a portion of the valve member from inlet pressure, and has an effective bellows area equal to valve seat nominal opening area.