A volume reducer for a hydraulic system, particularly useful for low pressure hydraulic systems such as used in engine compression braking systems. The hydraulic system has an actuator and a pressurizer, with a hydraulic line interconnecting the actuator and the pressurizer. A substantially straight section of the hydraulic line is provided, and a slidable body is provided in the substantially straight section. The body slides in the hydraulic line in response to pressure changes in the hydraulic line, and occupies hydraulic line volume, to stiffen the hydraulic system and reduce system sluggishness.