Abstract: A system for monitoring and controlling the level of liquid in a closed cainer includes a block for mounting at an upper end of the container, a lower level electrode for extending from the block vertically into the container, an upper level electrode for extending from the block vertically into the container and shorter than the lower level electrode, a valve for venting gas from the container, and a motor for driving the valve. Electric circuitry is responsive to liquid in the container rising to a free end of the upper level electrode to activate the motor to close the valve to increase gas pressure in the container to force lowering of the level of the liquid, and further electrical circuitry is responsive to the liquid in the container dropping below a free end of the lower level electrode to activate the motor to open the valve to vent the container and reduce gas pressure therein to permit rising of the liquid level.