Fix a cistern leaking into the pan by replacing the worn siphon diaphragm or valve seal inside, with the water isolated.
CN-01Check the leak and identify your cistern typeLift the cistern lid and watch where the water is going. A steady trickle into the pan when you haven't flushed almost always means the flush mechanism inside the cistern has failed, not the pan itself. Check whether you have an older black plastic siphon with a lever, or a newer push-button valve unit with a dual-flush plate - this decides which spare part you need to buy before you start.
CN-02Turn off the water and empty the cisternTurn off the isolation valve on the small pipe feeding the cistern - it's usually a slotted screw or lever near the base. Flush the toilet and hold the handle down to clear as much water as you can, then sponge out what's left in the bottom.
CN-03Disconnect the water supply pipeUndo the tap connector nut where the supply pipe joins the base of the cistern, using an adjustable spanner. Keep a cloth handy for the last splash of water that always seems to escape. Loosen it just enough to swing the pipe clear - don't force a stiff nut, a squirt of penetrating oil helps.
CN-04Remove the cistern from the panUnderneath the cistern, undo the two fixing bolts holding it to the pan, or to the wall brackets if you have a low-level suite. Lift the cistern straight up with both hands - it's heavier than it looks - and set it down on an old towel on the floor so you don't crack it.
CN-05Take out the siphon or flush valveTurn the cistern upside down on the towel. Unscrew the large retaining nut on the underside that holds the siphon or flush valve in place - some undo by hand, others need the spanner. Lift the whole unit up and out through the top of the cistern.
CN-06Fit the new washer or diaphragmOn a siphon, pull the perished black diaphragm flap off the bottom of the plunger and clip on a new one of the same size. On a valve-type cistern, replace the black rubber seal at the base of the flush valve instead. Make sure the new part sits flat with no gaps or twists.
CN-07Rebuild the siphon and refit the cisternPut the siphon or valve back into the cistern and tighten the retaining nut by hand, then just a quarter turn with the spanner - overtightening can crack the plastic. Lower the cistern back onto the pan, fit the fixing bolts, and tighten them evenly a little at a time, not fully one side first.
CN-08Reconnect the water supplyReconnect the tap connector nut to the base of the cistern and tighten it snugly with the spanner. Turn the isolation valve back on slowly and check the joint straight away for drips.
CN-09Test the flush and check for leaksLet the cistern fill fully, then flush it two or three times. Watch closely for a minute afterwards to check no water is trickling into the pan. Dab a folded tissue against each joint you touched - a damp mark means it needs another quarter turn.
CN-10Tidy up and dispose of old partsWipe down the cistern, pan and floor to catch any splashes. Bag up the old siphon or washer and put it in your normal household waste - it isn't recyclable.