The flush mechanism on your toilet consists of two valves. One is a supply valve that lets water into the tank when the level goes down, and the other is a flush valve between the tank and the bowl. When the flush valve is working properly, a rubber flapper prevents water from draining out. But when the rubber flapper flush valve wears out, slow leaking can cause the supply valve to cycle on and off, creating a disturbance and wasting water. In some cases you can stop a leak by replacing the flapper, but in others you may need to replace the valve seat.
Replace the flapper if it appears worn out or distorted and isn't making a good seal. Turn off the water supply, flush the toilet to drain the tank, disconnect the chain that connects it to the flush handle and pull the rubber ears off the overflow tube. You can use a universal flapper replacement for most valve models. Fit the ears onto the overflow tube, connect the chain and turn on the water.
Replace the entire valve if you can't get it to stop leaking after replacing the flapper. To do this, you'll have to disconnect the tank from the bowl.
Turn off the water and flush the toilet. Hold the handle down to drain as much water as possible, then sponge out the rest and transfer it to a bucket.
Unscrew the water supply hose from the bottom of the tank. You may be able to do this manually; if not, use slip-lock pliers. If you've left any water in the tank, it will spill out when you remove the hose, so place a bucket under the tank to catch it.
Unscrew the two bolts holding the tank to the bowl, using a screwdriver. You may need a helper to hold the nuts under the tank steady with a wrench while you turn the screws. When the screws are out, lift the tank off the bowl.
Pull the rubber washer off the threaded pipe extending from the centre of the tank and unscrew the large retaining nut with slip-lock pliers. When the nut is off, pull the valve out of the tank.
Fit the rubber seal that comes with a replacement flush valve around the threaded pipe at the bottom of the new valve and push it up as far as it will go. Push the valve into the hole in the tank and screw on the retaining nut from underneath with slip-lock pliers. Hold the valve steady as you tighten the nut.
Place the rubber washer that came with the replacement around the exposed end of the threaded pipe and set the tank in position of the bowl. Place the two bolts that hold it in the holes, being sure they have rubber washers around their heads. Slide rubber washers up onto the threaded ends of the bolts and screw the nuts on by hand from underneath. Have a helper hold the nuts steady with a wrench while you tighten the bolts with a screwdriver. Tighten each bolt incrementally, then tighten the other one to make sure the tank remains balanced on the bowl.
Fit a new flapper onto the overflow tube, if there isn't one already installed, and connect the flapper to the chain so it raises when you push the flush handle. Connect the water supply, turn it on and fill the tank. Test the flush and adjust the length of the chain if necessary.
Some flush valves will work only with special flappers. If you're in doubt, take the old flapper to the hardware store to find a suitable replacement. When you take out the old flush valve, be sure to clean the opening thoroughly before installing a new one. It's a good idea to use new bolts to attach the tank to the bowl.
Don't tighten the bolts holding the tank to the bowl too tightly, or you may crack the porcelain.