a high pressure pump system for use with a hydraulic engine system , such as a fuel injection system or a compression release brake system , provides variable delivery of pressurized fluid using sleeve metering principles . the relative position of metering sleeves with respect to pumping pistons is controlled electro - hydraulically by a control circuit . the control circuit receives pressurized fluid from the pump delivery gallery or another high pressure area and , using a pressure reducer , reduces the operating pressure within the control circuit to a substantially constant pressure lower than the pump output pressure . lower operating pressure within the control circuit improves the manufacturability of the control circuit components and helps to achieve better control of the pump output .