How to Fix a Running Toilet: Step-by-Step

Water UK has identified leaking toilets as one of the largest sources of wasted water in UK homes, so this is worth doing rather than living with. This is the repair guide. If you have not yet worked out which part is failing, start with our guide to diagnosing a toilet that keeps running — it takes two minutes and tells you which of the repairs below you need.

Once you know, the fix itself is straightforward. None of it requires a plumber, and none of the parts are expensive.

Before You Start

Isolate the water

Find the small valve on the pipe feeding the cistern, usually with a screwdriver slot. Turn it a quarter turn so the slot lies across the pipe. If there is no valve, use the main stopcock.

Empty the cistern

Flush and hold the handle or button down to let as much out as possible. Then sponge out what is left. Working in standing water makes every step harder and you cannot see what you are doing.

Tools

• Adjustable spanner or grips
• Flat and cross-head screwdrivers
• Sponge and a bowl
• Old towels
• The replacement part — take the old one to match

Repair 1: Adjusting the Float

The quickest fix, and no parts needed. Use this when the water is filling above the overflow.

1. Identify your float type. A ball on an arm is the older design; a cylinder sliding on a rod is the modern one.
2. Float arm: turn the small adjustment screw where the arm meets the valve, a quarter turn at a time. On older brass arms you can bend the arm gently downward instead.
3. Float cup: squeeze the clip on the side and slide the cup down the rod a few millimetres.
4. Turn the water back on, let it fill, and check the level. Aim for about 25mm below the top of the overflow.
5. Repeat in small increments. Overshooting downward gives you a weak flush instead.

Repair 2: Replacing the Flush Valve Seal

The most common repair, and the one that stops the most water waste.

1. With the cistern empty, look at the flush valve — the large plastic assembly in the middle.
2. Most modern valves release with a twist: turn the top section anticlockwise and lift it out. Some have a retaining clip instead.
3. Turn it over. The seal is a large rubber ring or disc on the underside.
4. Before replacing it, try cleaning it. Run a finger round the rubber and the seat it lands on. Grit or limescale under the seal is a very common cause, and cleaning costs nothing.
5. If the rubber is hardened, split or visibly distorted, prise it off and fit the new one, making sure it seats evenly all the way round.
6. Refit the valve, turn the water on, let it fill, and watch the pan for half an hour without flushing.

If you cannot get the valve out, the cistern may need lifting off the pan — at which point it is worth replacing the whole flush valve rather than just the seal, since you are already there.

Repair 3: Replacing the Fill Valve

Use this when the valve will not shut off cleanly and adjusting the float has not helped.

1. With the water off and the cistern empty, undo the supply connection underneath the cistern. Have a bowl ready — there is always some water left in the pipe.
2. Undo the back nut holding the valve shank through the cistern base and lift the old valve out.
3. Take it with you to buy the replacement. Shank length, thread and whether it is bottom or side entry all vary.
4. Fit the new valve with its washers in the right order — usually a rubber washer inside the cistern and a plastic one outside. Hand-tighten the back nut, then a small nip with a spanner.
5. Reconnect the supply, turn the water on slowly, and check for weeps at both connections.
6. Set the float level and test.

Do not over-tighten the back nut. Cistern bases are ceramic and they crack.

Repair 4: Adjusting a Dual-Flush Button

Common and often free to fix.

1. Remove the flush plate — most unclip or twist off.
2. Press each button and watch the valve underneath. It should lift cleanly and drop fully back.
3. If it does not return, the push rods are too long or misaligned. Most are threaded — shorten them slightly and retest.
4. Check the plate is not sitting so tight that it holds the buttons partly depressed.

Testing Properly

The Chartered Institute of Plumbing and Heating Engineering recommends verifying a cistern repair rather than assuming it has worked. Do the colour test.

Put a few drops of food colouring into the filled cistern and leave it thirty minutes without flushing. If no colour reaches the pan, the flush valve is sealing. If colour appears, water is still passing and the seal is not seated.

Also listen. A properly working cistern is silent between flushes. Any hiss, trickle or intermittent refill means something is still passing.

When to Replace Rather Than Repair

• Both the fill valve and flush valve are original and over ten years old — replace both together and save dismantling twice.
• The valve body is cracked rather than the seal worn.
• Parts for an older cistern are no longer available.
• You have replaced the seal and it fails again within months, which usually means the seat itself is damaged.

If the leak is escaping onto the floor rather than into the pan, that is a different problem — see our guide to a leaking toilet cistern.

Frequently Asked Questions

Do I need to turn the water off at the mains?

Only if there is no isolation valve on the toilet supply. Most cisterns fitted in the last twenty years have one within reach.

Can I just clean the seal instead of replacing it?

Very often yes. Grit and limescale under the seal are among the most common causes, and a clean fixes it. Replace only if the rubber has hardened or deformed.

How do I know which replacement part to buy?

Take the old one with you. Cistern components are not standardised and photographs are rarely enough to match them accurately.

My cistern still refills every few minutes after the repair.

Water is still escaping somewhere. Do the colour test — if colour reaches the pan, the flush valve seal is not seated properly. If it does not, look at the fill valve and the overflow.

If It Keeps Coming Back

A cistern that leaks again within weeks of a new seal usually has a damaged seat, and that means replacing the valve assembly rather than the rubber. We cover Milton Keynes and the surrounding area for plumbing repairs. Get in touch and tell us what you have already replaced.