The toilet that runs after flushing: flapper, chain, float, overflow

A toilet that hisses after the flush is usually losing tank water in one of two directions: past the flush seal into the bowl, or through the overflow because the fill valve is not stopping at the set level. Both faults are visible with the lid off and both are cheaper to diagnose before replacing the entire mechanism.

The ten-minute test

1. Watch the overflow. Lift the cistern lid while it refills. The water should stop below the top of the overflow tube. If it continues to pour into the tube, adjust or replace the float and fill valve according to the mechanism's instructions; do not bend an unfamiliar plastic arm until it cracks.
2. Do the dye test. Put a few drops of food colouring into the tank without flushing and wait. Colour appearing in the bowl means the flush seal or flapper is leaking. A chain that is too tight, a warped seal or scale on the seat can hold it open by a fraction that the eye cannot see.
3. Check the chain and handle. The handle should return freely and the chain should have a little slack. A chain caught under the flapper, a loose handle or a button that stays pressed keeps the seal from seating. Shut the small supply valve before putting hands deep into the tank.
4. Replace like for like. Take the old seal or valve to the parts counter and match its shape and connection. Universal parts exist, but the tank's thread, button and overflow arrangement still need to agree. Keep the old pieces until the new flush has been tested for leaks.

When the tank is not the problem

A toilet that refills after another fixture runs may be responding to low supply pressure, a shared tank or a valve elsewhere. A bowl that rises, drains slowly or gurgles moves this page into the blockage and drain branch chapter; do not keep changing the flapper while the waste line is restricted. Close the supply valve if the toilet cannot stop running and the overflow is near its limit. The small leak is not harmless because it is hidden: the dye test turns the sound into a location, and the location tells you whether a seal, a valve or a drain professional owns the next move.