Abstract: In this invention, a breather device for a tank comprises a closed reservoir above the container but having a conduit which depends into the container and has an opening which determines liquid level in the container. The container comprises air inlet means which will allow entry of air into the container from above liquid level, and a breather tube which opens into the container below liquid level (or vice versa) so that liquid normally closes the air flow path to the breather tube, but the depth of water level is so small that a slight pressure differential only is required to cause sufficient displacememt of water to open that air flow path.