First, find out where the water is going
There are two kinds of leak and they are fixed differently: water escaping into the bowl (nothing on the floor, but the toilet keeps running or refilling) and water escaping outside (dripping under the cistern or pooling on the floor).
- Food colouring test: add a few drops to the water in the cistern and don’t flush for half an hour. If the water in the bowl changes colour, the flush valve is leaking.
- Paper test: stick a piece of dry toilet paper to the inside of the bowl just under the rim. If it gets wet, there is a constant trickle into the bowl.
- If the floor is wet, dry everything and run paper along the joints underneath. The first one to get damp shows you the source.
The two parts that matter: fill valve and flush valve
A standard cistern has two mechanisms. The fill valve (grifo or válvula de llenado), with its float (flotador), lets water in and shuts it off at the right level. The flush valve (descargador or válvula de descarga) is the central column that lifts when you press the button; a rubber seal (junta) at its base keeps the water in.
Lift the lid and look. If water is constantly spilling into the overflow tube, the fill valve is the problem. If the level is below the overflow but the cistern still tops itself up now and then, the flush valve seal isn’t closing properly.
If the toilet keeps running: adjust the float
- Look for a water level mark on the overflow tube or the cistern wall. The water should stop a couple of centimetres below the top of the overflow.
- On modern vertical fill valves, lower the level with an adjusting screw or wheel, or by sliding the float down its rod.
- On older valves with a ball float on an arm, bend the arm slightly downwards or use the screw at the pivot.
- If the valve still won’t shut off with the level lowered, its diaphragm is worn or scaled up with limescale. Cleaning can help, but replacing the whole fill valve is usually the reliable fix.
If it leaks into the bowl: replace the flush valve seal
Over time the seal hardens, warps or gets coated in limescale, which is common with Madrid tap water too, and stops sealing. Turn off the small isolation valve on the wall next to the toilet (llave de escuadra), or the main stopcock (llave de paso) if there isn’t one, then flush to empty the cistern.
On most dual-flush mechanisms the central part comes out with a quarter turn or by releasing a clip, without taking the cistern off. Clean the seat, fit a new seal of the same model and take the old one to the hardware shop (ferretería) to match it. If the button sticks or the mechanism is broken, replace the whole flush valve.
If it drips underneath or the floor is wet
- Flexible hose (latiguillo): the braided pipe from the isolation valve to the cistern. If it drips at the threads or looks rusty, replace it; it is a cheap, quick part.
- Isolation valve (llave de escuadra): if it leaks at the handle or won’t close fully, it needs replacing, which means turning off the main stopcock first.
- Cistern-to-bowl seal: on close-coupled toilets, a large rubber seal and the fixing bolts join the cistern to the bowl. If it drips there when you flush, the cistern has to come off to replace them.
- Water at the base of the toilet only when flushing can point to the connection with the waste pipe. In summer, drops on the outside of the cistern may just be condensation.
Concealed cisterns, your water bill and when to call
A cisterna empotrada is a concealed cistern built into the wall (Geberit and similar brands). You get in through the flush plate: remove the plate and its frame, and behind it are the cistern’s own shut-off valve and the fill and flush mechanisms, which come out through that opening. Parts are brand-specific, so identify the model before buying anything.
Don’t leave it: a cistern that runs constantly can waste hundreds of litres a day, and it shows on your water bill. Adjusting a float or replacing a flush valve, hose or isolation valve are small jobs we usually do in one visit. Send us a photo of the inside of the cistern or the flush plate and we’ll quote in English with no obligation. If you rent, our guide Who pays for repairs? explains who usually covers it.