Toilet cistern keeps running: how to find the leak and fix it

What you need

• Small adjustable spanner or 10 and 13 mm spanners
• Flat and Phillips screwdrivers
• Flat-nose pliers
• Sponge, cloth and bucket
• Phone torch
• Food colouring or a blue cistern dye tablet
• Flush valve seal for the mechanism (original Roca or universal Wirquin seal, 4 to 8 euros)
• Complete flush mechanism if it is more than 10 years old (Roca dual flush, Geberit Impuls or Wirquin Tronic, 20 to 35 euros)
• Foam rubber cistern-to-pan seal and bolt kit with rubbers (6 to 12 euros at Leroy Merlin or Bricodepot)
• Cleaning vinegar for the limescale
• Steel braided flexible hose 3/8 inch by 1/2 inch if the current one is rusty

Step by step

1. 1

   Do the dye test

   With the cistern full, put the colouring in the tank and do not flush. Wait 15 minutes. If the water in the pan changes colour, the leak is internal: flush valve or level too high. If the pan stays clean, look outside with dry kitchen paper around the base and the hose.

2. 2

   Close the isolating valve and empty the cistern

   Turn the isolating valve on the wall clockwise until it stops. Flush and mop up the remaining water with the sponge. If the valve will not close fully, shut off the mains for the flat.

3. 3

   Remove the lid and check the level

   On a dual push button, unscrew the button ring anticlockwise or press both buttons and turn. On a pull button, unscrew the button. Lift the lid and look at the overflow tube: the water mark should sit 1.5 or 2 cm below it. If it reaches the rim, lower the float with the adjustment screw or clip on the fill valve.

4. 4

   Take out the flush mechanism

   Almost all are bayonet fit: turn the body a quarter turn anticlockwise and lift. On Geberit there is a side tab to press. Turn it over and you will see the rubber seal at the base, usually deformed or scaled up.

5. 5

   Clean the seat and replace the seal

   Wipe vinegar with the sponge over the plastic rim where the seal sits and leave it 10 minutes. Fit the new seal of the same diameter, without twisting it. If the mechanism is more than 10 years old or the body is yellowed and brittle, replace the complete mechanism and adjust its telescopic height so the overflow sits 2 cm below the lid.

6. 6

   If it leaks outside, replace the connecting seal

   Disconnect the hose, loosen the nuts on the two fixing bolts from under the pan and lift the cistern off. Remove the old foam rubber, clean the ceramic and fit the new one centred on the flush opening. Reassemble with new bolts and rubbers, tightening by hand and then half a turn alternating sides.

7. 7

   Reassemble, turn the water on and check

   Refit the bayonet mechanism, the lid and the button. Open the isolating valve slowly. Let it fill, flush twice and repeat the dye test. Check the floor with dry paper after two hours.

Why the cistern keeps running: the three possible leaks

A cistern can only leak in three places. The first and most frequent is into the pan: you hear a constant trickle, a limescale line forms on the ceramic and the water meter never stops. The fault is the rubber seal on the flush valve, which hardens over the years and stops sealing, or the fill level, which rises above the overflow tube so the excess water runs into the pan.

The second place is outside, between the cistern and the pan. You will see drops at the back of the toilet, damp on the floor or marks on the wall. Here the failure is the connecting seal (a thick foam rubber or a conical washer) or the two fixing bolts with their rubbers, which loosen or rust.

The third is the water inlet: the flexible hose, the isolating valve or the fill valve nut. You can tell because the damp is at the lower side of the cistern, right where the pipe comes in. In summer, in coastal flats, there is a false positive: the cistern sweats with condensation from the cold water and looks as if it is leaking. If the drops are spread over the whole outer surface and disappear at night, there is no leak.

• Trickle in the pan and a limescale line: flush valve or float set high.
• Puddle behind the toilet: cistern-to-pan seal or fixing bolts.
• Damp at the side or bottom inlet: flexible hose, isolating valve or fill valve.
• Drops all over the cistern only in summer: condensation, not a leak.

Roca cistern leaking: what usually fails

Most toilets in Valencia and Alicante are Roca: Victoria, Dama, Meridian, The Gap or Debba. When a Roca cistern leaks, it is almost always the bottom seal on the dual-flush mechanism, which deforms with limescale. Roca sells the seal on its own for about 5 euros and the complete mechanism for 20 to 35 euros. If your cistern is more than ten years old, replace the whole mechanism: a new seal on a worn body starts dripping again within months.

To remove the Roca dual push button there is no screw: unscrew the chrome ring anticlockwise, or press both buttons at once and turn. We have a dedicated guide in how to repair or replace a Roca cistern mechanism with photos of each model.

Common mistakes when fixing a dripping cistern

Mistake number one is replacing the seal without cleaning the valve seat. Limescale forms a crust on the plastic rim where the rubber sits, and the new seal does not seal. Wipe it with a sponge and cleaning vinegar and leave it to work for 10 minutes before fitting.

The second mistake is dropping the float to minimum so it stops overflowing. It works, but the flush is so short that the toilet does not clear properly and you end up flushing twice. The correct level is 1.5 or 2 cm below the rim of the overflow.

The third is tightening the fixing bolts hard to stop an external leak. Ceramic does not give: it cracks. Tighten by hand and then half a turn with the spanner, alternating sides. If it still drips, the connecting seal is badly positioned or the wrong size.

• Do not use PTFE tape or silicone on the flexible hose: the rubber washer seals on its own.
• Do not mix brands: a universal mechanism fits almost everything, but the flush seal has to be the exact diameter.
• Do not leave the isolating valve half open: over the years it seizes and then will not close when you need it.

When to call a professional

Call a plumber if the leak is external and the connecting seal does not fix it, if the mechanism is a discontinued model and you cannot find a replacement, if the isolating valve will not close, or if the cistern is concealed (Geberit, Roca In-Wall) and the work has to be done through the push-button opening. Also if you rent the flat out and need an invoice with IVA (VAT) for the landlord.

Our plumber service in Valencia and Alicante replaces the cistern mechanism for 60 to 110 euros, with the mechanism included in most cases, in a single visit of 30 to 45 minutes. We give you a fixed price by WhatsApp with a photo of the open cistern, and we arrive in 20 to 45 minutes in Valencia, Alicante, Torrevieja and Benidorm. If the cistern is cracked or is a model with no spares, replacing the whole toilet costs 90 to 150 euros in labour. All our rates are in how much a plumber charges. 6 months written guarantee and payment on completion.

Limescale and hard water on the coast: why your cistern leaks sooner

The water in Alicante, Valencia and on the Costa Blanca is hard. Limescale builds up on the flush seal, on the valve seat and in the fill valve filter. That is why mechanisms here last 5 to 8 years and not 15 as in soft water areas. A cistern leaking a constant trickle wastes 100 to 400 litres a day; at water prices in the Valencia region, that is 10 to 30 euros a month down the pan.

Three habits extend the life of the mechanism: use the short flush whenever you can, clean the valve seat with vinegar once a year and do not put chlorine tablets in the tank, because they degrade the rubbers within a few months. If your home is empty for part of the year (a holiday home), close the isolating valve when you leave: you stop a slow leak turning into a water bill of hundreds of euros.

If the cistern is also slow to fill, the problem is usually the fill valve filter blocked with limescale: we explain it in toilet cistern not filling or filling slowly.