A running toilet can drive anyone insane, plus it’s a huge drain on the environment. Whether you hear water running constantly or cycling on an off, it is time to stop the leak.
Test the flapper
First shut off the water supply valve under the toilet. Flush the toilet to drain out most of the water and unhook the old flapper. Buy a new flapper of the same type from your local hardware store, and install it according to the instructions. Hook the flapper chain onto the flush lever arm so there is a slight slack when the flapper is closed. If the flapper doesn’t leak and the water is still running, check the fill tup connected to the overflow pipe. The end should be above the water line. If the end is under the water, cut it back.
Next, inspect the fill valve for visible signs of ear and test the float. If it is improperly adjusted, the water level can rise above the overflow pipe and drain into it. Replace the old film valve if it doesn’t shut off or papers the floor-arm operation. Install a new floatcup-style fill valve. Adjust the float according the the instructions. Attach the flapper chain to the flush lever with slight slack, turn on water and test flush.