An improved mechanical dump valve for a liquid level control system. The dump valve comprises a splined shaft that communicates with the valve actuator by a toothed member, such as a splined bore in a dump valve hub. The dump valve also may include pressure ports. Still further, the present invention includes a liquid level control system comprising a trunnion assembly and a mechanical dump valve in which the trim motion of the float valve is transmitted by means of splined shafts in both the trunnion assembly and the dump valve assembly. The inventive system may comprise float weights for weighting the float accurately without having to drill the float and load it with sand or other weighting material. Also included is a kit for retrofitting existing liquid level control systems that use D-shaped connections with splined components.