A continually running toilet usually suffers from a leak at the flush valve. The most common flush valve component to fail is the flapper, an oblong rubber plug that raises and lowers with every push of the tank's lever. Damaged flappers allow a constant trickle of water to escape into the toilet's bowl and, although the tank appears full, it's continually replenishing the lost water and adding dollars to your utility bill. Fortunately, flappers are among the easiest toilet parts to replace, so even the novice plumber can quickly repair a toilet that's not shutting off when the tank is filled.
Turn the toilet tank's water supply valve to the "off" position.
Remove the tank's lid and set aside. Flush the toilet and allow water to drain from the toilet tank.
Unlock the clasp that attaches the flapper's chain to the toilet's flush handle. The chain is located at the end of the bar attached to tank's handle. Follow the chain to its connection with the flapper. Unhook the chain from its connection to the flapper.
Slide the flapper's hooks or rubber tabs off of the posts that connect the flapper to the toilet tank's flush valve. Remove the flapper from the toilet tank.
Insert a replacement flapper into the toilet tank. Place the flapper over the toilet tank's flush outlet. Position the replacement flapper's hooks or tabs facing toward the flapper's attachment posts. Slide the flapper's hooks or tabs onto the posts.
Attach the replacement flapper's chain to the flapper. Attach the opposite end of the chain to the flush handle's bar.
Turn on the toilet's water supply valve. Allow the tank to fill to capacity. Press the tank's flush handle. If the tank does not fully flush, the chain is too loose. Tighten or "choke-up" the chain on the bar until pressing the handle results in a full flush.
Replace the toilet tank's lid.
Flappers are not designed to uniform standards. To find the right flapper for your toilet, remove your old flapper and take it to the hardware store when you’re buying repair materials. Use the old flapper to find a match for your toilet tank.