High Quality Content by WIKIPEDIA articles! A ballcock (also balltap or float valve) is a mechanism or machine for filling water tanks, such as those found in flush toilets, while avoiding overflow and (in the event of low water pressure) backflow. The modern ballcock was invented by Joseph Bramah and Thomas Twyford.. The diagrams and description ONLY apply to the USA (these are not typical in the UK or Europe). It consists of a valve connected to a hollow sealed float (literally, a device that floats on top of water) by means of a lever, mounted near the top of the tank. The valve is connected to the incoming water supply, and is opened and closed by the lever which has the float mounted on the end. When the water level rises, the float rises with it; when the level rises to a pre-set level (called a fill line), the mechanism forces the lever to close the valve and shut off the water flow. This is an example of negative feedback and of proportional control.