Toilet Keeps Running? How to Stop a Cistern That Won't Stop Filling

A toilet that never quite stops — a constant trickle into the pan, or the cistern refilling every few minutes — can waste a surprising amount of water. It's nearly always one of two parts: the flush valve or the fill valve. Lift the cistern lid and you can usually tell which.

First: where's the water going?

• Trickling into the pan (you can see water running down the back of the bowl): the flush valve isn't sealing.
• Running out of the overflow pipe outside, or into the pan via the overflow tube in the cistern: the fill valve isn't shutting off — the water level is too high.

Fix 1: The fill valve (water level too high)

1. Look at the water level. It should sit below the overflow — usually marked with a line inside the cistern.
2. Most modern fill valves have an adjustment screw or clip on the float. Lower the float so the valve shuts off earlier.
3. If it still won't shut off, the valve's washer or diaphragm may be worn, or there may be debris in it. Turn off the isolation valve on the supply pipe, unscrew the top, clean it, and replace the washer or the whole valve.

Fix 2: The flush valve (leaking into the pan)

1. Push-button cisterns use a drop valve with a rubber seal at the bottom. Over time it hardens, collects limescale or gets grit trapped under it.
2. Turn off the water, flush to empty the cistern, then twist or unclip the valve to lift it out.
3. Clean the seal and the seat it sits on. If the rubber is cracked or misshapen, replace the seal — they're cheap, but take the old one with you to match it.
4. Older lever cisterns with a siphon use a flap valve inside, which also wears out and can be replaced.

Quick test for a slow leak

Put a few drops of food colouring in the cistern and don't flush for 30 minutes. If the colour shows up in the bowl, the flush valve is leaking.

When to call a plumber

• The cistern itself is cracked or leaking from the bolts underneath.
• The parts are concealed behind a panel or wall-hung frame.
• The isolation valve won't turn or is leaking.
• You've replaced the seal and it still runs.

We fix running toilets all the time, and fit new ones too. Typical costs are in our plumber cost guide. For a hand in Preston, Bamber Bridge, Chorley or nearby, call 07949 075721 or use the form below.