exceed the fill line, the water will pour into the overflow tube and out to the bowl (in the flapper valve type) or to an outside drain (in the siphon type). Although this does not stop the wastage of water, it avoids the flooding that would otherwise occur after a single-point valve failure. Typically the fill valve is secured by a ballcock shank washer, which prevents leakage from the supply line. These devices are sold at most hardware, plumbing supply, and home improvement stores. Delay valves are becoming more common in the UK, which delay the filling until the level has dropped to a low level. This avoids short-cycling of the water supply. References External links Introduction to ballcock valves ; hysterisis issue. "Such flush tank valves as now commonly used are closed against the pressure of water from a source of supply which pressure opposes the movement of the float under rising water level in the tank. This results in a slow or delayed movement of the valve preliminary to its reaching its fully closed position and accordingly develops an objectionable hissing sound due to the discharge of water through the then slightly open valve under full water pressure; this delayed action continuing until the slowed delivery of water to the tank finally raises the water level to a point where the float will be effective in fully closing the valve. This action of the conventional float controlled valves frequently gives rise to a condition where full closing of the valve does not occur with a consequent continuous leakage of water through the valve with a resulting waste of water." ; hysterisis issue. Plumbing valves Toilet components