Abstract: Provision is made for removing gaseous impurities, such as air, from hydraulic fluid in a hydraulic power system. A sump tank serves as a reservoir for the hydraulic fluid which passes through the power unit in a closed loop manner. The fluid inlet and outlet lines are connected to the tank below the level of hydraulic fluid therein. A source of vacuum is connected to the tank above the fluid level for removing impurities from the fluid. In a preferred embodiment, a flexible member is used to adjust the volume of the evacuated area of the tank as a function of the pressure in the tank space above the fluid level. The tank may further include one or more adjustable expandable devices which are used to regulate the fluid pressure within the system as desired.