How to Fix a Running Toilet – Illustrated Guide

The two most common causes for a toilet leaking into the bowl are a worn-out flapper and the other one is the fill valve which might not be stopping to fill.

A toilet that keeps running after you flush it is really annoying. Mine has done it for a while and it usually stops if I tap the toilet a bit but that’s not really a fix it’s just a temporary solution. Luckily this is a perfect DIY project that you can fix yourself without too many tools.

Diagnosing the problem

Why is the toilet running and water is leaking into the toilet bowl? It’s quite easy to figure out is the problem with the flapper or the fill valve. If you close the water intake and the toilet still keeps running then you know it’s the flapper leaking and not the fill valve overfilling the tank.

If the fill valve makes an annoying loud whistling noise when filling up then it’s best to go ahead and replace the fill valve.

Fixing the problem

In the below chapters I go through three possible fixes depending on which issue is causing your toilet to leak into the bowl.

Replacing the fill valve

I had the issue that when I flush the toilet the water was overflowing via the overflow tube into the toilet bowl because the fill valve didn’t stop filling.

You can first try to flush debris from the fill valve as described in a chapter further down below. In my case, this didn’t help so the way to go was replacing the fill valve.

In the above video, I show you the steps to diagnose the problem and how to remove the old fill valve and install a new one.

By doing the repair yourself you will likely save around 50 to 150 $. A plumber or handyman will either charge an hourly rate or a flat fee based on the job required.

Steps for installing a Fluidmaster fill valve

I bought the Fluidmaster PRO45B fill valve because it a brass metal shank and therefore should last longer. The other Fluidmaster models like the 400 series models look pretty much the same and have the same installation steps.

Steps for installing a Fluidmaster fill valve:

  1. Remove the lid from the tank
  2. Turn off the water supply to the toilet
  3. Disconnect the pipework / water supply
  4. Remove the old fill valve from the toilet tank (cistern)
  5. Place the sealing washer on the threaded shank
  6. Adjust the height of the Fluidmaster valve to match the old one
  7. Install the new Fluidmaster valve inside the tank
  8. Place the nut on the threaded shank
  9. Reconnect water supply and tighten the nut
  10. Turn on the water supply to the toilet
  11. To complete the installation clear debris from the waterline. See the video at the end of the article on how to do this.

Most of the steps are simple and the only tools I needed was a wrench.

Where to buy the fill valve

My toilet has a Fluidmaster 400 fill valve. I live in Finland and the price for one of those is 69,90€ in a local Bauhaus store. I think that’s crazy expensive. Luckily ships to Finland and the price is less than a third. even has free shipping to Finland if you order over a certain limit. If you are in the US the price is much lower and can be even under 10 $. For the current pricing check the links below.

Replacing the flapper

The second most common reason for a running toilet is the flapper wearing out. Often the flapper becomes worn out due to mineral deposits since it’s submerged in water.

The below video discusses diagnosing an issue with the flapper. Replacing a flapper is fairly easy and a universal flapper will only cost around 5$.

Flushing debris from a Fluidmaster fill valvle

Flushing debris from water line with Fluidmaster fill valve

If the fill valve is not stopping to fill the tank, fixing it might be as easy as cleaning the fill valve seal as shown in the below video flushing it with water.

The above diagram explains how to flush debris from the Fluidmaster fill valve.

  • To open the cap assembly you need to first lift the arm up and then rotate the cap anti-clockwise.
  • Next, you put a container or cup over the top of the fill valve and let the water run for 30 seconds clearing any debris.
  • Reassemble by placing the cap back on and rotating clockwise.

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