The problem of a toilet that constantly runs can become an around-the-clock headache. Water that runs continuously inside your toilet is the result of a water intake valve, or ballcock, that fails to shut off the incoming fresh water at the end of the flush cycle. The problem of continuously running water can be caused by a float device that has come improperly adjusted, a waterlogged ballcock or a defective tank ball, but most running toilets are the result of a worn flapper or ball.
Inspect the float ball to see if it is submerged in the water. If the float ball is more than halfway below the surface of the water, the problem is a waterlogged ballcock that is too heavy to rise enough to shut off the valve. Replace the float ball to fix this problem.
Check the guide arm if the problem isn't a float ball. Ensure that the guide arm is not bent so that it catches on the inside of the tank or the overflow tube. Straighten out the guide arm to fix this cause of a running toilet.
Bend the guide arm to adjust the water level. If this doesn't work, turn the adjustment screws located on the fill valve. Flush the toilet and check the water level again. When properly adjusted, the water level will become even with the fill line on the tank or with the overflow tube.
Turn off the water supply and flush to empty the toilet. Open the fill valve and clean the interior parts. Replace the seals if any of them have become worn.
Use an abrasive pad or emery paper to clean the valve seat. Clean the flapper or ball with a sponge. Lift a worn flapper off the lugs and hook on the replacement flapper if necessary.
Disconnect the water supply to replace the valve seat. Remove the bolts that secure the tank to the bowl using a screwdriver and pair of pliers.
Turn over the tank and remove the large, round spud washer. Remove the locking nut on the flush valve with a spud wrench.
Install the new parts and reverse the process to reattach the tank. Turn on the water supply and check to make sure that the toilet no longer runs.
Place a few drops of food colouring into the tank to see if you have a leak from the tank into the bowl.
Some products used to sanitise toilets contain chlorine. Chlorine can damage the rubber parts inside a toilet like the seal and float balls and, therefore, should be avoided.