A cistern that runs is usually the washer on the valve
I heard the loft overflow in April, a thin trickle into the gutter that did not stop. The toilet had filled. The inlet was still letting water in, and the extra was leaving by the overflow. I isolated the cistern at the valve on the supply, tested the tap in the same room so I knew I had the right pipe, and took the lid off. The float was up. The little washer in the inlet valve was split. Water was passing a shut valve. I put a new washer in, opened the isolate, and the cistern filled and stopped. The china was fine. The disc was not.
A fill valve lets water in until the float rises and shuts it. The shut is a washer or a diaphragm on a seat. If that disc is split, hard, or missing a flake, the valve never quite closes. The cistern runs. The overflow drips. People replace the suite. I isolate, lid off, washer, lid on. If the valve body is cracked, I stop. A cracked body will not hold a new disc.
An isolation valve that will not close is usually the slot is how I shut the supply. I look at the line, I turn it across, I listen. If the cistern is still filling, I am not isolated. I do not start. If I cannot isolate, I stop. I do not take a valve apart on a live feed. A tap that drips is usually the washer in the head is the same disc habit on a different fitting. A bottle trap that weeps is usually the washer is the waste under the basin, not this tank.
When I check I look at the water line. If it is up at the overflow and still moving, the inlet is passing. I isolate, I drain a little with a flush so I can see the valve, and I take the inlet apart only as far as the washer. I match the old disc. I clean the seat. Grit on the seat is a run you will put back. I do not bend the float arm into a pretzel to hide a bad washer. A little height change is fine once the disc actually seats. A bent arm on a split washer still runs.
Skip a new cistern because the overflow dripped. Skip dropping the lid into the bowl. I put it on the floor, out of my feet. Skip forcing a plastic cap with grips until it cracks. That is the stop I already named. Skip bleach tablets sitting against the valve if they have eaten the rubber. Skip PTFE on this washer. It has to sit on the seat. Skip a full bathroom refit for a disc I can hold between two fingers.
Some valves are a sealed cartridge. If I cannot get to a washer, I replace the valve, not the suite, and only if I am still isolated. I will not invent a brand. I will say I take the old one with me.
Night is when you hear it, or a wet afternoon when the overflow stains the wall. I now listen after a flush. Fill, then quiet. If it never quiets, I isolate before I buy china.
I do not need a new toilet because the cistern ran. I need the supply shut, a washer that seats, and a stop if the valve body has split.
If I later add a buy link here, it will be a paid promo.