This happens when water leaks through the hole at the bottom center of the tank under the flapper because it does not seat properly. Sometimes the flapper chain gets twisted, snagged, or caught on the handle arm, which shortens the chain’s length, holding up the flapper so it can’t seat fully. The ideal chain length has some slack when the handle arm is at rest and the flapper is fully seated. Make sure the flapper chain is untangled.
Flush the toilet a few times to assure the chain does not re-tangle and the flapper seats properly. There have been cases of corroded handle arms, rusted chains, and broken links. You can use string, a twist tie, and a paper clip to temporarily repair flapper chains. Later, replace the old chain with a new one.
Considering the flapper itself, there have been cases of flappers so old some of its rubber has been eaten away by water; the rubber loop the chain hooks into has disintegrated; and, one or both of the hinges have eroded or merely slipped off its posts. To repair or replace the flapper:
Test the Flapper – With tank lid off, handle at rest, and toilet seat up, push down the flapper. If the water stops running into the bowl, the flapper needs to be repaired or replaced.
Test the Chain and Hinges – Make sure the chain end is fastened securely to the flapper loop, so the flush handle operates the flapper correctly. Make sure the flapper hinges are attached securely onto the posts or bottom overflow tube, so the flapper floats up and down freely.
If the flapper still does not seat correctly, then install a new flapper.
Shut off the Water – Turn the shut-off valve clockwise to shut off the tank’s water supply. Remove tank water by flushing the toilet.
Remove the Old Flapper – Disconnect the chain from the top of the flapper. Disconnect the flapper’s hinges from their posts. Remove the old flapper.
Install the New Flapper – Attach the chain to the new flapper and set the flapper in place, attaching it to the hinge. Connect the chain to the flush handle arm. Turn the water back on.
Running Overflow Tube
Sometimes, tank water is flowing over the top of the overflow tube. Check the tank water level; optimal level is ½-inch below the top of the overflow tube. If water flows into the top of the overflow tube, then the float assembly’s adjustable float is not working properly. Readjust the float by increasing the length of the rod connecting the float to its arm. If the water continues to overflow, replace the adjustable float.
Turn the Water Off – Turn the shut-off valve to shut off the tank’s water supply. Remove tank water by flushing the toilet, scooping out leftover water, and drying with rags.
Remove the Old Float Assembly – Loosen and remove the nut at the bottom of the float assembly at underside of the tank. Unclip the float arm from the overflow tube and remove the old float assembly.
Install the New Float Assembly – To install a new float assembly, reverse the above steps. Clip the float arm to the overflow tube and secure the nut at the bottom of the float assembly at underside of the tank. Turn the water back on and adjust the float to the proper depth.