A valve system for controlling a liquid level in a vessel, the valve system comprising: a container (10, 52, 104) comprising at least one flexible wall (19, 108) and is at least partially filled with liquid (12, 111); a housing (14, 44, 102) confining the container, the housing comprising at least one aperture (18, 50) in fluid communication with an interior of the vessel through which liquid can enter from the vessel into the housing (14, 44, 102) to a level corresponding to a desired level of liquid in the vessel, the housing being sized and configured to allow said at least one flexible wall (19, 108) of the container (10, 52, 104) to expand and contract laterally resulting from a combined effect of a pressure exerted on said at least one flexible wall from an inside of the container (10, 52, 104) by the liquid (12, 111) in the container due to a gravity force and a pressure exerted on said at least one flexible wall from an outside of the container by the liquid in the housing due to a buoyant force; a normally closed valve (22, 40, 100) comprising an inlet connected to a source of liquid and an outlet connected to be in fluid communication with the interior of the vessel; an actuator mechanism (20, 56, 112, 116) for the normally closed valve (22, 40, 100) comprising a first actuator part (24, 58, 120) configured to be moved and retracted, thereby, respectively, opening and closing the normally closed valve, and a second actuator part (28, 66, 116) configured to apply a force to move the first actuator part responsive to a lateral expansion and a lateral contraction of said at least one flexible wall (19, 108); the second actuator part (28, 66, 120) being configured to apply no force to the first actuator part (24, 58, 120) allowing the first actuator part to retract when the buoyant force acting on said at least one flexible wall (19, 108) is at a value corresponding to the vessel being at a desired full level, and to apply a force sufficient to move the first actuator part (24, 58, 120) to open the normally closed valve (22, 40, 100) when the buoyant force acting on the container (10, 52, 104) is at a value corresponding to the vessel needing to be refilled. a container (10, 52, 104) comprising at least one flexible wall (19, 108) and is at least partially filled with liquid (12, 111); a housing (14, 44, 102) confining the container, the housing comprising at least one aperture (18, 50) in fluid communication with an interior of the vessel through which liquid can enter from the vessel into the housing (14, 44, 102) to a level corresponding to a desired level of liquid in the vessel, the housing being sized and configured to allow said at least one flexible wall (19, 108) of the container (10, 52, 104) to expand and contract laterally resulting from a combined effect of a pressure exerted on said at least one flexible wall from an inside of the container (10, 52, 104) by the liquid (12, 111) in the container due to a gravity force and a pressure exerted on said at least one flexible wall from an outside of the container by the liquid in the housing due to a buoyant force; a normally closed valve (22, 40, 100) comprising an inlet connected to a source of liquid and an outlet connected to be in fluid communication with the interior of the vessel; an actuator mechanism (20, 56, 112, 116) for the normally closed valve (22, 40, 100) comprising a first actuator part (24, 58, 120) configured to be moved and retracted, thereby, respectively, opening and closing the normally closed valve, and a second actuator part (28, 66, 116) configured to apply a force to move the first actuator part responsive to a lateral expansion and a lateral contraction of said at least one flexible wall (19, 108); the second actuator part (28, 66, 120) being configured to apply no force to the first actuator part (24, 58, 120) allowing the first actuator part to retract when the buoyant force acting on said at least one flexible wall (19, 108) is at a value corresponding to the vessel being at a desired full level, and to apply a force sufficient to move the first actuator part (24, 58, 120) to open the normally closed valve (22, 40, 100) when the buoyant force acting on the container (10, 52, 104) is at a value corresponding to the vessel needing to be refilled.