How the Toilet Tank Works
The toilet tank's function is to hold a quantity of water until you flush the toilet, at which time the water in the tank rushes down through an opening in the bottom of the tank and into the bowl, forcing waste out of the bowl and into the home's drain and sewer lines. There are two major toilet parts in the tank that make this possible: the flush valve and the fill valve.
The toilet fill valve is the mechanism that fills the tank with water. It is also known as a "ballcock" or a "refill valve." The fill valve is usually located to the left side of the tank as you look down from above with the tank lid removed. Fill valves comes in four basic variations:
Plunger-type ballcock: the oldest type, usually made of brass
Diaphragm-type ballcock: older styles may be brass, newer types are plastic
Float cup fill valve: a newer design, usually made of plastic
Floatless fill valve: another newer design; not allowed by some codes
Whatever the design, the fill valve works to automatically open the water supply valve when the water level falls in the tank during a flush, then shuts off automatically when the water level rises to a specific level in the tank. Depending type of design, the valve is operated either by a floating ball or a float cup that moves up and down with the water level in the tank. Floatless fill valves operate by sensing water pressure at the bottom of the tank.
If you remove the tank lid and watch what happens inside the tank during the flush cycle, you will quickly understand the mechanics of how a toilet flushes.