Abstract: An improved overfill shut-off system for a liquid storage tank includes a passage having an inlet end and an outlet end, a valve seat in the passage, a plunger that is movable within the passage for the purpose of engaging the valve seat to impede flow of liquid in the passage, and a guide member for guiding movement of the plunger. The plunger and the guide member are sealed with respect to each other and together define a control space. A first port is defined in the plunger for communicating the control space with an upstream end of the passage, and a second port is defined in the guide member for communicating the control space with a downstream end of the passage. A valve is provided for selectively impeding fluid flow through the second port in response to the level of a float member within the storage tank. When liquid level in the storage tank reaches a predetermined maximum, liquid flowing through the passage will be permitted through the first port, but not through the second port.