Guide · Drains and toilets
Toilet cistern that keeps running without flushing
If a thin stream of water keeps running into the bowl, or the cistern refills by itself every so often, there are two culprits: the float that does not shut off the supply, or the flush seal that does not hold. Close the cistern's isolation valve, look at the water level against the overflow, and you will know which one to fix. These are screwdriver jobs, even on concealed cisterns.
30-60 minutes Difficulty: MediumUpdated 25 October 2026
What you need
- Flat screwdriver
- Food colouring
- Sponge and bucket
- Gloves
- Phone to photograph the model and parts
Step by step
Tap a step to mark it as done.
The toilet cistern float: types and adjustment
The float controls the fill valve, the tap that lets water into the cistern: when the level rises, the float lifts and shuts it. In old cisterns it is a ball at the end of an arm. In newer ones it is a cylinder that slides on a vertical rod next to the valve.
If the level is set too high, the extra water leaves through the overflow and runs into the bowl without you pressing anything. Lowering it a little is usually enough. If the level is right and water keeps coming in, the fill valve is clogged with limescale or has a worn diaphragm, and it needs cleaning or replacing.
Geberit flush that does not fill with water
In Geberit concealed cisterns the float is part of the float valve, mounted at the side with its water connection. If the cistern does not fill, or takes very long, first check the isolation valve behind the plate: after some work it can be left closed or half open.
The second suspect is the filter at the inlet of the float valve, which catches sand and limescale and gradually clogs. On many models you reach it by disconnecting the inlet pipe from the float valve, with the isolation valve closed, and rinsing it under running water. If you cannot see how to get to it without taking other parts off, stop and photograph the mechanism.
To buy the right spare you need the model. Geberit tells its cisterns apart by name, years of production and the size of the opening behind the plate. The 12 cm Sigma, for example, has a 22 by 14 cm opening. Geberit advises against descaling the inside of the cistern, because pieces of limescale that break off can block the flush.
Toilet leaking water into the bowl: the other causes
Sometimes the seal is fine but the flush valve does not drop all the way down. It happens when a button linkage is too short, when a rod is bent or when the button gets stuck in the plate. Press the button with the cistern open and watch whether the valve drops freely.
A cistern that leaks makes the meter run day and night, even if the floor stays dry. If your bill has gone up and you see no water in the house, the colouring test is the first check to do.
When to stop and call a technician
- There is damp around the plate or a stain on the floor below: the concealed cistern may be leaking inside the wall
- The exposed cistern drips from the bottom or from the pipe that connects it to the bowl
- The cistern isolation valve does not close or leaks when you turn it
- The whole mechanism of a concealed cistern has to be replaced and you cannot find the spare for your model
- A plate or plastic support cracked while you were taking it apart
Frequently asked questions
Why does my toilet cistern leak water without flushing?
Almost always it is the flush valve seal not closing, or the float letting in too much water and sending it into the overflow. The water level in the cistern tells you which one.
How do I adjust the toilet cistern float?
On arm floats you adjust the rod. On vertical ones, a screw or clip lets them slide. The water must stop below the top of the overflow.
My Geberit flush does not fill with water: what do I check?
First the isolation valve behind the plate, then the filter at the inlet of the float valve, and finally that the float is not stuck. If everything is fine and the cistern stays empty, the float valve needs replacing.
How do I adjust the Geberit float?
For the type 380 float valve, the Geberit data sheet says the filling height is adjusted by turning the float pivot. For other models follow the instructions or the level mark. In every case the water must stop below the top of the overflow.
How much does it cost to repair a toilet cistern?
On quote websites in the Naples area, a toilet repair usually costs roughly 50 to 400 €. A seal costs much less than a complete concealed mechanism. We cover Campania, and a video call is often enough to work out which part to buy.
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