The five minute test
Take the cistern lid off and set it somewhere safe, they crack easily. Now look at the water level. If water is sitting above the overflow tube, or above the marked fill line, and it is running down that tube, the fill valve is not shutting off. If the water level is correct and steady but water is still moving into the pan, the flush valve seal at the bottom of the cistern is passing.
If you cannot tell, wipe the back of the pan dry and put a piece of toilet paper against the porcelain just below the cistern outlet. If it wets, water is passing the flush valve. The old dye trick works too: a few drops of food colouring in the cistern will show up in the pan within a few minutes if the seal is leaking.
Fill valve faults
Fill valves fail in predictable ways. The diaphragm washer perishes, so the valve never quite closes. Limescale builds on the seat, which is very common in Gloucestershire hard water areas, and holds the valve fractionally open. On older float arm types the arm bends or the float ball takes on water and sits too low to shut the valve off.
On a float arm valve, bending the arm down slightly is a legitimate free adjustment. On a modern side or bottom entry valve there is usually a screw or clip adjustment for the shut off height. If the valve is scaled or the washer has gone, replacement is the sensible answer. It is an inexpensive part and a job of around half an hour with the isolation valve turned off first.
Flush valve and dual flush faults
Dual flush mechanisms are the single biggest cause of running toilets in modern homes. The seal is a large rubber washer that sits on a plastic seat, and grit, scale or a slightly warped seat means it never seats fully. The leak is often silent, which is why people discover it on a water bill rather than by hearing it.
Check the seal for scale, ridges or splits, and check that the flush button or lever is returning fully rather than resting slightly depressed. A button that stays part pressed holds the valve open. Seals are sold to match the mechanism, so photograph the unit before buying and take the old seal with you if you can.
Safe steps before you start
Turn off the isolation valve on the pipe feeding the cistern, usually a slotted screw on a small in line valve, and flush to empty. Have a towel and a bowl ready because there is always some water left. Do not force stubborn plastic backnuts, they crack, and a cracked cistern is a far bigger job than the part you were replacing.
If the cistern is concealed behind tiling or in a furniture unit, check what access you have before you begin. Some concealed frames need the flush plate and an inspection panel removed, and a few need the pan pulling to reach anything.
When it is worth calling someone
Call if you have replaced the obvious part and it still runs, if the cistern is concealed and you cannot reach the valve, if the porcelain is cracked, or if water is appearing on the floor rather than only inside the pan. A slow floor leak around a WC will rot a floor quietly.
We repair and replace WC mechanisms as a small standalone job across Gloucester, Cheltenham, Tewkesbury, Stroud, Newent and the Forest of Dean. Tell us the make of the flush plate through the enquiry form and we can usually bring the right seal first time.