Abstract: A fluid reservoir for hydraulic power systems comprises a tank divided into a liquid compartment and an air compartment by a flexible diaphragm so that the liquid will be pressurized by compressed air in the air compartment. The tank is supported in a stationary frame upon horizontal pivots and a balancing spring. The liquid inlet and outlet connections pass through the pivots. An indicator is provided for showing the vertical position of the tank relative to the frame on the balancing spring and thus indicating the amount of liquid in the tank.