Running Toilet Diagnosis: Why It Will Not Stop

A cistern is a simple machine. Water comes in through an inlet valve, sits in the tank, and leaves through an outlet valve when you press the button. When a toilet will not stop running, one of those two valves is misbehaving, and a two-minute check tells you which.

Take the lid off the cistern and look inside. If the water level keeps rising until it spills into the overflow tube, the vertical pipe in the middle of the tank, the inlet valve is not shutting off. If the water level keeps dropping and the inlet keeps topping it up, water is escaping into the bowl past the outlet washer. A trickle down the back of the bowl that never quite stops is the classic sign of the second fault.

The inlet valve is controlled by a float. As the tank fills, the float rises and progressively closes the valve until, at the set level, it shuts off completely. Two things commonly go wrong. First, the washer or diaphragm inside the valve wears out, so even with the float fully up, water keeps seeping through. Second, the float itself sticks, catches on the flush mechanism, or is set too high, so the valve never gets the signal to close.

Grit is a factor here too. Melbourne water is generally kind to fittings, but any works on the street mains can send a slug of sediment through the line, and a single grain lodged in the valve seat is enough to keep it weeping. In the older homes we service around Brighton and Gardenvale, corroding galvanised pipework upstream can shed rust flakes that do the same job.

A quick test: lift the float gently by hand. If the water stops, the valve mechanism works and the float setting is the problem. If water keeps flowing with the float held right up, the valve seals are done and the valve needs servicing or replacing.

The outlet valve sits at the bottom of the cistern and holds the stored water back with a rubber washer or seal. Rubber hardens, distorts and grows a film of mineral scale over the years, and once the seal is imperfect, water trickles constantly into the bowl. The inlet valve then tops the tank up in short bursts, which is the intermittent hissing or filling sound people notice at night.

This fault is sneaky because it can waste a large amount of water without ever looking dramatic. If you want to confirm it, put a few drops of food colouring in the cistern and wait twenty minutes without flushing. Colour appearing in the bowl means the outlet seal is leaking.

On button-flush cisterns the culprit is usually the seal on the bottom of the flush valve cartridge. On older lever-style cisterns, still common in unrenovated Edwardian and Californian bungalow bathrooms across Bayside, it may be a flapper or a bell washer, and on some heritage high-level cisterns the parts are imperial sizes that no longer match anything on the hardware shelf.

In Victoria, most plumbing work must legally be carried out by a licensed plumber, but there are sensible things a householder can do with a cistern. Adjusting the float level, cleaning grit off a valve seat, and swapping a like-for-like washer or flush seal inside the cistern are generally regarded as basic maintenance. Turn off the small isolation tap on the wall beside the toilet first, flush to empty the tank, and take the old part to the shop so you match it exactly.

Where we would tell you to stop: anything involving the water connection to the cistern, replacing the inlet valve itself, replacing the cistern or pan, or any work where the toilet leaks at the base rather than inside the tank. A leak at the base can mean a failed pan collar or a cracked pan, and that is sewage territory as well as licensed work. The same goes if the fix does not hold, because a running toilet that resists a new washer usually has a worn valve seat or a misaligned mechanism that needs a proper look.

Plenty of the toilets we see in Brighton and Hampton are decades old, and some of the parts inside them left production years ago. We carry seals and valves that cover most of what is out there, and our first preference is always the honest repair. But when a cistern is on its third mismatched washer, the ceramic is crazed, or the flush uses twice the water of a modern dual-flush unit, replacement is the better answer and we will say so.

A modern dual-flush suite uses far less water per flush than the single-flush units fitted before the 1980s, so on a toilet that gets real daily use, the change pays for itself in water saved and callouts avoided. If your bathroom is heritage in character, there are traditional-style suites that keep the look without keeping the plumbing problems.