How Check the Toilet Flapper Valve
The flapper valve is located in the bottom of the tank. It seals the tank drain until the handle is pressed. When the flush handle is pressed, a linkage from the handle to the flapper valve pulls the stopper up and releases the water into the bowl. If this seal is not tight, leakage may occur. The leakage can result in the toilet unexpectedly refilling or making steady dripping or running water sounds.
Remove the tank lid. Turn off the water valve located under the tank. Flush the toilet to drain out the water. The flapper valve is at the bottom of the tank, usually in the middle and is connected to the handle by a chain or other linkage.
The flapper valve should be clean and smooth. If the flapper has a build up of minerals or algae, has a rough surface or has become warped, it will not make a tight seal with the tank drain. The tank drain also should be clean and smooth. If there is a build up, try scrubbing the flapper and drain with a rag dampened with bleach.
Make sure there is some slack in the linkage from the handle to the flapper. If the linkage is tight, it might hold the flapper slightly open. If there is too much slack in the linkage, it may get between the flapper and drain and prevent it from sealing.
Some flappers attach to a collar that can slide up the overflow tube, which prevents the flapper from sealing. Slide the collar all the way down so that the flapper can seal properly.
If the flapper and drain cannot be cleaned or the flapper is damaged or warped, then the flapper should be replaced. Usually, replacing just the flapper will do the trick, although, in some cases, both the flapper and the drain seat must be replaced.