When a flush valve causes a toilet to run, a worn flapper is usually the culprit. But not always. First, look at the chain that raises the flapper. If there’s too much slack in the chain, it can tangle up and prevent the flapper from closing firmly. A chain with too little slack can cause trouble too. Photo 3 shows how to set the slack just right.
1. Test the flapper. Push down on the flapper with a yardstick and listen. If the sound of running water stops, the flapper needs replacing.
Turn off the water, flush the toilet to empty the tank and then run your finger around the rim of the flush valve seat. If you feel mineral deposits, clean the flush valve seat with an abrasive sponge or ScotchBrite pad. Don’t use anything that might roughen it. If cleaning the flush valve seat doesn’t solve the problem, you need to replace the flapper.
2. Remove the old flapper from the ears of the overflow tube and detach the chain from the handle arm.
3. Attach the new flapper to the overflow tube and hook the chain to the handle arm. Leave 1/2 in. of slack in the chain. Turn the water back on and test-flush the toilet.