The clack valve is a one-way flow valve incorporated into a pump system to prevent the water from flowing back down the pump pipes (rising main). There was a hinged flap inside the clack valve which closed onto an iron seat, and this flap is usually faced with leather to form a watertight seal. The name is said to derive from the noise made as the flap closed. A number of cast iron clack valve housings can be seen on the Wheel Wreck.