How to Fix a Leaking Toilet Cistern – A Complete Guide

Cisterns are water storage tanks of different sizes. These are used to store water for different purposes. In many areas, rainfall water is stored in cisterns to be used later on. However, in plumbing terms, a cistern is alternatively used as the toilet tank. The purpose of the toilet tank is also to store water and then release it when needed.

In old-fashioned toilets, cisterns or tanks are connected high above the toilet bowl. In modern toilet designs, cisterns are placed close to the toilet bowls. This new setup is known as close coupling.

Understanding of Cistern is Necessary:

Sometimes, the cistern gets out of order and needs to be fixed immediately to avoid any further damage. If you do not know the working philosophy of the cistern, you will never be able to fix any issue in it.  It is required because it reduces the dependency on a plumber. In addition to that, there are some emergencies that you should be able to deal with.

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So if you want to fix your toilet cistern but do not know anything about it, this guide will explain everything that you must know. After explaining the working principle, the fixing process of a cistern is elaborated.

The Components of a Toilet Cistern:

There are three main components of a toilet cistern that are listed below:

  •         Fill valve
  •         Supply line
  •         Float mechanism

We will briefly explain the working of these components before moving on to the functioning of a cistern.

Supply Line:

The cistern base is attached to the supply line. The water from the supply line fills the tank or cistern. If the supply line is disturbed or damaged, the cistern will contain no water at all. The supply of water is controlled through a fill valve in the cistern.

Fill Valve:

The fill valve is a controlling switch for the water coming in through the supply line. As far as the fill valve is open, the water flows into the tank. Once the valve is closed the water stops entering the tank. The valve is in synchronism with the float mechanism.

The Float Mechanism:

The float mechanism monitors the level of water and based on that, it allows the fill valve to open to close. The threshold level is predefined for which the fill valve is opened or closed. Once the water in the tank has crossed the threshold level, the float mechanism allows the fill valve to close and shut the supply of water into the tank.

Working Principle Of a Cistern:

The cistern helps the water to reach the toilet bowl for flushing purposes with the help of gravity or air pressure. The user activates the flush by pushing the button. In some of the modern toilets, the touch-free option is also available. 

The flush process activates the float mechanism. The float mechanism then allows the inlet valve to start filling the tank again. In this way, the tank starts filling once again and when the water level is enough to close the fill valve, the float mechanism allows the fill valve to close.

How to Fix a Cistern?

Now we assume you have gained enough knowledge about the different components and working philosophy of toilet cisterns. Let us move on to the fixing procedure of a toilet cistern. There are normally two issues that are not noticed in the toilet cisterns:

  •         Overflow of water when not needed
  •         Does not flush when required

Fixing The Overflow Problem


Incorrect Setting Of The Valve:

Overflow problems usually result from the incorrect setting of the inlet valve. Try to locate the float and move it in a clockwise direction to raise the water level and in an anti-clockwise direction to lower the water level. In this way, you can change the threshold water level and prevent overflow in a toilet cistern.

How To fix Toilet Cistern

Damaged Valve Or Float:

Another cause of overflow can be the damaged valve or float. In this case, the control mechanism is not there to check the incoming water quantity. To fix this issue you will need to call a professional.

No Flushing When Required


Supply Line Issue:

If the cistern does not flush, check the supply line first. If the water does not flow in the tank how would the cistern flush? If the supply line has some issue, fix it yourself or call a plumber. If the supply line is correct but the threshold water level is set too low, the fill valve will not allow the water to come in. Try to adjust the float by rotating to a reasonable water level setting.

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Malfunctioning Valve:

The Second issue that can cause the tank to not flush is the malfunctioning valve. The valve may not be opened even if the float is allowing it to be. In this case, you will need to replace the valve. 

Damaged Cistern:

Another issue associated with no flushes can be the damaged tank itself. If the tank leaks the water and gets empty, it will not only waste the water but also will be empty when required to flush.

Float Mechanism Issue:

If the float mechanism is not working properly, it cannot give instructions to the fill valve and hence there is a chance that the fill valve will remain closed after a flush and the cistern remains empty.


An overflowing or malfunctioning toilet cistern is a headache for homeowners. To fix the cistern you need to have some prior knowledge about its working and the key components. Once you get enough knowledge you can move on to the fixing process. All these domains are covered in this article.

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