How to Fix a Leaking Toilet Cistern

You need to fix your leaking toilet cistern. It can be annoying, noisy, and it can cost you money, but it’s not likely to be an expensive problem to fix.

If you have a leaking toilet cistern it is probably time to replace, or clean up the flush valve washer. The good news is that it can be simple to fix. So, before you call out a plumber, take a look at your toilet cistern to see if it’s something you can do yourself.

What Tools Will You Need to Fix a Leaking Toilet Cistern?

Most of the time the only tool you will need is a small, flat-headed screwdriver. You will use it to turn the water supply off at the valve next to the cistern.

Keep an old towel, or a rag on the floor beside the toilet to catch any drips.
All the working parts of a toilet cistern are either clipped or threaded together. You won’t need any complicated plumbing equipment to fix your leaking toilet cistern.

How to Find the Problem and Fix a Leaking Toilet Cistern

The first thing to do is to turn off the water supply to the toilet cistern.
Look below the cistern for the water inlet pipe. It is usually a flexible pipe with an inline feed valve fitted where it meets the copper or plastic main water supply.
If it is a thumb-turn, it is simple enough to twist it ninety degrees and shut the water off. If not, use your screwdriver to close off the supply.

Removing the Lid of the Toilet Cistern

With a modern push-button flush on your cistern there are two ways to remove the lid. The first, and most common method is to undo the ring around the dual-flush buttons, and then lift them out.
Lift off the cistern lid and put it somewhere safe. Ceramic cistern lids can chip and break if dropped onto a tiled floor.

Some dual flush cisterns can require you to depress one button to lift out the other to reveal a slotted screw head. Undo that screw with the flat-head screwdriver to release the cistern lid.

What You Will Find in the Toilet Cistern

There are two assemblages in the cistern, the float valve, which regulates the incoming water level, and the flush valve.
Empty the cistern by pushing down gently on the pad, or tube on top of the flush valve assembly. This action will activate the plunger at the bottom, and release water into the toilet pan.
Don’t worry. It’s all nice, clean water, which has come in directly from the mains. There will be no nasties in the cistern!

What to Check to Fix a Leaking Toilet Cistern

There are three checks you can make to the apparatus inside the cistern and they can quickly tell you what the problem is.
The likely culprit is the flush valve as it gets the most use. The float valve can also cause problems, as can water pressure. All three of these issues are easy to fix and require no special tools or know-how.

Check the Flush Valve

When you need to fix a leaking toilet cistern, the most common cause is that the flush valve is at fault. A simple check at this stage is to feel with your fingers whether any items are stuck in the opening below the plunger.

If your fingers come away with a dark smudge of black rubber, it is a sign that the outlet washer has perished and is the problem.

If your fingers come away with a white, crumbly material on them, it is a sign that limescale is the problem.

Check the Float Valve

Is the float valve set correctly? It’s simple to check.

If the water in the cistern is flowing directly down the overflow the float valve will require adjusting. The overflow is usually a vertical plastic tube that links directly to the flush assembly.

All you need to do is adjust the float level on the assembly so that the water level is lower than the overflow. In most cases, this will involve adjusting a plastic nut set just below the float.

You can spot the nut as is often a contrasting colour to the rest of the unit, and it can turn easily with your fingers.

Check the Water Pressure

Another straightforward check to make is water pressure. If the water entering the cistern does so at too high a rate, it can fill the toilet cistern faster than it can empty.

The incoming water forces the flush valve to remain open.

To fix the water pressure turn the feed valve down to restrict the flow of water into the cistern. Turn off the water supply and flush and empty the cistern. Let the flush valve close, and then open up the feed valve halfway.

Once the cistern has filled up, you can flush it again to see if this has fixed the problem.

The Flush Valve is Letting Water By

When you need to fix a leaking toilet cistern the most common problem is the washer on the bottom of the flush assembly.

The good news is that these are cheap to replace, and the even better news is that they may just need a clean.

If you are in an area where limescale can be a problem, it could be that you have scale on the valve washer preventing it from making a proper seal.

How To Fix a Leaking Flush Valve

  • The first thing to do is to empty the cistern.
  • Next, give the assembly a little wiggle. If it is threaded it should unwind anti-clockwise about a quarter turn before it releases from the bottom of the cistern.
  • If it won’t turn, feel for the clips that are holding it into the bottom of the cistern. Be gentle and don’t force it.
  • Lift it out of the cistern and turn the assembly upside-down and inspect the rubber washer seal on the bottom of the plunger.
  • You may notice that the washer, if intact, curls back toward the top of the assembly. This is quite normal.
  • Remove the retaining clip and prise off the washer.

What to do With the Washer

  • Sometimes the washer is in poor condition, pitted, or missing sections. If this is the case then you should replace it with one from your local big-box DIY store.
  • Limescale deposits on the washer can cause problems. These scrub off easily with soap, water, and a nylon nail brush.
  • Clean the washer and push it back onto the bottom of the plunger. Ensure that if a curl is apparent, it is oriented toward the bottom of the assembly. It will make for a better seal.

Putting Everything Back

  • Before you attach any retaining clips, use the flat-head screwdriver to make sure the washer is gently pushed home so the clip fits cleanly around the bottom of the plunger.
  • Reinstate the flush valve assembly so that all the clips catch home, or until the thread holds it back in the correct position.
  • Turn the water back on and let the cistern refill.
  • Engage the flush mechanism on top of the unit.
  • Watch, and listen, for any water movement in the toilet bowl after the flush has finished.
  • You know how to fix a leaking toilet cistern if the water in the toilet bowl is still, and silent. Well done! Time to reverse the first steps and put the cistern back together.
  • If water continues to leak into the toilet bowl, check that the washer has been refitted properly.

If You Cannot Fix a Leaking Toilet Cistern Yourself

You may need to have the flush valve assembly replaced if the surface in contact with the plunger’s washer is damaged. They are not expensive, and, as you may have discovered, not difficult to fit if you can find an identical model.

Less often, the float valve may have become worn, or damaged. Again, a replacement is inexpensive and straightforward to replace, but you may require a spanner.

If you are not confident with DIY, a registered plumber can fix a leaking toilet cistern. Expect a plumber to offer to change both the flush valve and float valve assemblies. This will guarantee a leak-free cistern and avoid a return visit.

If there is water leaking from the base of the cistern, it is likely that the washers coupling it to the toilet pan have become perished. The cistern will require decoupling from the pan to replace them.