A valve mechanism is described for controlling fluid flow, particularly for quickly adjusting the flow between a full flow position and an accurately defined limited flow position. The valve has a cylindrical valve member which is slidably mounted in a central bore and this valve member has a region of reduced diameter with an annular wall extending between the region of reduced diameter and the cylindrical surface of the valve member. One of these annular walls forms a sharp corner with the cylindrical surface and a series of notches are provided in this sharp corner. A pair of flow connected annular recesses are provided in the bore holding the valve member so that at full flow, the region of reduced diameter is over both annular recesses while for restricted flow fluid connection between the annular recesses is via the notches. A third position is also possible where the valve member completely closes the connection between the annular recesses thus preventing any flow.