Blog · 26 September 2026 · 4 min read
Toilet Keeps Running After Flush: Causes and How to Fix It
Short answer: a toilet that keeps running has one of two faults. Either the fill valve does not shut off (water keeps coming in) or the flush valve seal leaks (water keeps escaping into the bowl). Lift the cistern lid and watch for a minute — it tells you which one, and both are usually fixed by replacing a single part.
It is worth fixing quickly: a running cistern can waste hundreds of litres a day, and it is one of the most common reasons for an unexpectedly high water bill.
Step 1: find out which fault you have
Take the lid off the cistern and look at the water level and the overflow pipe (the open tube in the middle or at the side).
Water rises above the overflow and pours into it: the fill valve is not closing.
Water stays below the overflow but you see a trickle into the bowl, or the cistern refills by itself every few minutes: the flush valve seal is leaking.
The food colouring test confirms it: put a few drops in the cistern and wait 15 minutes without flushing. Colour in the bowl means the flush valve leaks.
Fixing a fill valve that will not shut off
Close the small isolation valve on the pipe that feeds the cistern, then flush to empty it.
Many fill valves can be adjusted: lower the float slightly (by a screw or a clip, depending on the model) so the valve closes earlier.
Limescale and grit often stop the valve sealing. Most modern valves have a removable top — clean the seal and the small filter at the inlet.
If it still runs after cleaning and adjusting, replace the fill valve. It is a standard part for most exposed cisterns.
Fixing a leaking flush valve
At the bottom of the flush mechanism is a rubber seal that closes the outlet to the bowl. Over time it hardens, deforms or collects limescale, and water seeps past it.
Cleaning the seal and the seat it rests on sometimes helps for a while. The lasting fix is a new seal, or a new flush valve if the mechanism is old or brittle.
Check the chain or cable too: if it is too short it holds the valve slightly open.
Concealed cisterns (wall-hung toilets)
With a wall-hung toilet the cistern sits inside the wall. You reach it by removing the flush plate — it usually unclips — and sometimes a frame behind it.
The faults are the same, but access is tight and parts are specific to the brand (Geberit, Grohe, Roca and others). It is usually a job for a plumber, and it does not require breaking the wall. See toilet and cistern repair.
Other signs your cistern needs attention
A weak flush that needs two goes, a button that sticks, a cistern that takes a long time to refill, or a hissing noise at night. All point to worn parts, and fixing them together saves a second call-out.
Water on the floor behind the toilet is a different problem: usually the connection hose, the isolation valve or the seal between cistern and pan.
How much water it wastes
A slow seep might waste a few dozen litres a day; a fill valve pouring into the overflow can waste several hundred. Water that passes the meter is billed to you, so a running toilet shows up on your bill before anywhere else. See average water bill in Portugal.
Dual-flush buttons that stick
Most modern cisterns have a dual-flush button (small and large flush). If the button or its linkage sticks, the flush valve stays slightly open and water keeps running.
Take the lid off and press the button slowly: watch whether the valve drops back fully. Scale, a misaligned rod or a cracked button assembly are common culprits. Adjusting the rod length or replacing the button often solves it.
The hissing cistern
A constant faint hiss usually means the fill valve is letting a little water through all the time, or refilling because the flush valve seeps. Put your ear to the cistern at night — if you can hear water, you are paying for it.
Finding the right parts
Exposed cisterns mostly use universal fill and flush valves, available in hardware shops. Take the old part with you, and note the height of the cistern and the type of button.
Concealed cisterns use brand-specific parts. Note the brand on the flush plate or the inspection opening — Geberit, Grohe, Roca and others — before buying.
When to replace the whole toilet or cistern
A cracked cistern, a porcelain toilet that wobbles because the fixings have rotted, or a very old low-level cistern with obsolete parts are good reasons to replace rather than repair.
Repairs start from 60 €. For replacement we quote after seeing photos. See toilet and cistern repair.
Concealed cistern, or the fix did not work? We repair cisterns and flush mechanisms, from 60 €.
Toilet and cistern repairFrequently asked questions
Can I just close the valve until it is fixed?
Yes. Close the small valve on the pipe feeding the cistern and open it only to refill before flushing.
Is it worth replacing both valves at once?
If the cistern is old and both are worn, yes — it avoids a second repair a few months later.
Why does my toilet refill by itself at night?
The flush valve is leaking slowly; the fill valve tops the cistern up when the level drops.
Do I need to break the wall for a concealed cistern?
Almost never. Everything is reached through the flush plate opening.
Who pays in a rented flat?
Wear of the mechanism is usually the owner's responsibility. Tell your landlord in writing.
How long does a cistern repair take?
Usually under an hour for an exposed cistern; a concealed one can take a little longer because of access.
Services Lisbon plumbing team
Guides written by the plumbers who do this work in Lisbon homes every week — drains, leaks, water heaters and pipework.
