If a toilet continues to run after the flush cycle is complete, it may be because the ballcock is failing to shut off the supply of fresh water. A continuously running toilet can waste more than 20 gallons of water every day, according to “The Complete Guide to Home Plumbing.” In most cases, making a simple adjustment to the ballcock will solve the problem.
Turn off the water supply to the toilet. Some toilets have a shut-off valve behind their tank. Alternatively, turn the water off at the mains.
Remove the cover to the toilet tank and empty it by flushing the toilet. Dry the ballcock assembly with a towel.
Find the screw at the highest point of the assembly and use a small amount of petroleum jelly to lubricate it so that it can be moved easily.
Turn the screw half a turn clockwise to lower the ball. Turn the water back on and flush the toilet to test whether the adjustment is sufficient. If not repeat steps 1 to 3 until water stops flowing after the flush cycle is complete. In older toilets without an adjustment screw, bend the float arm downward slightly to lower the water level. This method also works in newer toilets when large adjustments need to be made.