The plumber quoted me two hundred eighty dollars to fix a toilet that would not stop running. I had already replaced the flapper. Twice. I was convinced the toilet was haunted. It was not. It was the fill valve — a twelve-dollar part and fifteen minutes of work.

Rule Out the Easy Stuff First

Lift the float arm in the tank. If the water stops, the flapper is the problem. If the water keeps running even with the float lifted all the way, the fill valve has failed. This is the distinction every YouTube video skips past.

Replacing the Fill Valve

Turn off the water supply at the wall valve. Flush the toilet to drain the tank. Unscrew the supply line from the bottom of the fill valve — have a towel ready for the remaining water in the line. Underneath the tank, there is a plastic nut holding the fill valve. Unscrew it. The old valve lifts out.

New universal fill valves cost about twelve dollars and come with instructions. The key adjustment is the height — set it so the top of the valve is about an inch below the top of the overflow tube. If it is too high, water spills into the overflow constantly. If too low, the tank does not fill enough for a complete flush.

The Flush Valve Seat

If replacing the flapper did not fix the issue, run your finger around the flush valve seat — the ring the flapper sits on. Mineral buildup creates an uneven surface that the flapper cannot seal against. Scrub it with a scouring pad (not steel wool — it rusts) until smooth. I did not do this the first time and replaced a perfectly good flapper.

Chain Slack

If the chain between the flush handle and flapper is too short, it holds the flapper slightly open and water leaks silently. You will never hear it. Your water bill will notice. The chain should have just enough slack that the flapper closes fully but the handle still lifts it when you flush. About half an inch of slack is right.