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Flush-tank mechanisms consist of two control valves. Levers and other parts open or close these valves at the proper time.

One of these valves is the flush valve which is seated at the bottom of the tank. This is kept closed by a rubber flush ball, diaphragm or flapper. When the outside handle on the toilet tank is pressed down it raises a trip lever that pulls the flushing device off its seat. Water inside the tank pours through the opening to flush the toilet bowl.

The flushing device is held in place by water pressure; however, once the device is lifted by the trip lever, it remains off the seat by floating on top of the water until the tank is empty. As the water level drops, the flush ball or flapper gradually settles back into the opening, sealing it so the tank can refill for the next flush.

Some toilets have a trip lever with two-way action to conserve water. When pushed in one direction, a full flush occurs. When pushed the other direction, a partial flush results, which is adequate for water waste.

Standard toilets previously had 3.5 gallon tanks, but beginning Jan. 1, 1994, federal law mandated that toilets that use no more than 1.6 gallons per flush.

In their search for a method to increase flushing power, some manufacturers have introduced systems that harness compressed air to add greater velocity to each 1.6 gallon flush. These toilets hold water in a pressurized chamber until needed.

Refilling is controlled by the second valve mechanism, the inlet valve or ballcock. As the water level inside the tank drops, the hollow metal or plastic float ball drops, pulling the float arm down with it. This arm opens the inlet valve inside the ballcock and permits water from the supply line to pour into the tank through the ballcock.

As rising water forces the float ball upward, it gradually shuts off the inlet valve until the flow of water is stopped entirely when the tank is full.

If this valve fails to operate properly, the water does not shut off at the right time. Excess water then flows out through an overflow pipe, which stands vertically inside the tank.

A refill tube, which replaces water in the toilet bowl after it has been flushed, also squirts a small stream of water through the overflow pipe while the bowl is flushing. This refills a trap built into the toilet bowl to keep sewer gases from escaping into the house.


In some homes and many commercial buildings, a flush valve instead of a tank flushes the toilet bowl.

Flush valves may develop leaks around a vacuum breaker in the valve. It is often difficult to find which of several seals and/or diaphragms in the valve is leaking, so it is often best to recommend replacement of all internal parts. Flush-valve repair kits are available to facilitate replacements.


Ballcocks or toilet fill valves are sold in pre-assembled units; replacement parts are available from the manufacturer, also in prepackaged units.

Replacement parts include upper lever, float rod, lower lever, plunger, valve seat, refill tube, nylon seat, eye screw, body, hush tube, regular shank, shank gasket, lock nut, coupling-nut washer, riser pipe and repair shank.

In addition to ballcocks, several other devices perform the same functions.

One is a toilet fill valve that can be installed underwater. It measures the water level from the bottom of a toilet tank. As the toilet is flushed and water level falls, its internal diaphragm senses the weight of the water above it. As the water level drops, the reduced pressure opens the fill valve and supply water enters.

When incoming water rises to a predetermined point (usually 8″ deep), increased pressure on the diaphragm closes the valve.

This system is simpler than ballcock valves because it does not need a ball float or float rod and has fewer internal parts that can malfunction or wear out. These hydraulic systems, since they work with the water pressure, generally provide faster, more positive water shutoffs eliminating many of the noises associated with the older style filling valves.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.