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A cistern that's leaking from underneath should be looked at as soon as possible. Not only can a leaking cistern cause expensive water damage in your bathroom, but it will increase your utility bills by driving up water usage as well. There are a few different ways to fix a toilet cistern that's leaking underneath, depending on what part of the cistern is leaking.
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Look the cistern over to see if you can determine where the leak is coming from. A leak from around the supply hose connection could indicate a bad gasket or stripped coupling while a leak from the center of the tank near the bowl might indicate a leaking bolt gasket.
Turn off the water supply valve for the toilet cistern. This is usually situated either on the floor next to the toilet or the wall behind it. Grasp the knob on the end of the valve and turn it clockwise.
Flush the toilet then remove the cistern top. In dual-flush systems, this may require unscrewing the push-button trigger from the top of the cistern before the top can be removed.
Loosen the coupling that connects the supply pipe or line to the tank if this is the part of the cistern that leaks. Check the coupling gasket and replace it if necessary. Clean off the threads of the supply inlet with a piece of steel wool. Wrap the threads with plumber's tape then reconnect the coupling.
Loosen the nuts on the underside of the cistern with a wrench or pair of pliers if the cistern leaks between the bowl and the tank. Disconnect the supply line coupling by loosening it with a wrench. Lift the cistern up and off of the tank once the screws are removed.
Examine the cistern bolts. If the gaskets on top are cracked or otherwise damaged, purchase new ones of the same size.
Replace the cistern onto the back of the bowl. Insert new cistern bolts through the bottom of the cistern and through the bowl. Thread the nuts underneath hand-tight. Tighten them another turn or two with the wrench.
Reconnect the coupling and turn on the water supply.