A mechanism is provided for use with a hydraulic control mechanism of a hydraulic tool. The hydraulic control mechanism is attached to the hydraulic tool to provide a desired hydraulically powered function. The mechanism allows the hydraulic control mechanism to be used with either a constant volume hydraulic system or a constant pressure hydraulic system. The mechanism provides a valve chamber and a valve member positioned within the valve chamber. The valve chamber communicates with both a central passageway and a cross passageway of the hydraulic control mechanism. The valve chamber defines a valve seat proximate to one of the central and cross passageways. The valve member is displaceable within the valve chamber and is configured such that, depending on the position of the valve member within the valve chamber, the hydraulic control mechanism can be used with either a constant volume hydraulic system or a constant pressure hydraulic system.