Toilets leak at the bottom of the tank around the flapper valve or at the top of the tank at the overflow tube. To test the flapper valve, remove the lid from the back of the tank and mark the water level in your toilet tank with a pencil. Turn off the water supply to the toilet. If the water remains on the mark you made for 10 minutes, the flapper valve is not leaking. If the water level drops below the mark you made, the flapper valve is leaking and should be replaced.
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