Usually this is caused by one of the following problems: The water level in the tank may be at or above the top of the overflow pipe. The flapper or stopper ball that closes off the outlet at the bottom of the tank may be defective. The fill valve may need replacing.
Adjust the Fill Height by Checking the Float
The water level in the tank is controlled by an adjustable float. A float that's set too low produces a weak flush; if it's set too high, water spills into the toilet overflow tube and the fill valve won't shut off. The toilet keeps running.
If your Fluidmaster fill valve is not shutting off all the way or not filling quickly like it did when it was new, then replacing or cleaning the fill valve seal will most likely fix the problem.
Keep the float valve lowered.
A faulty fill valve is one of the major causes of a running toilet. This happens when the water level becomes excessively high, leading to water overflowing from the top tank and running into the overflow tube and, finally, the toilet bowl. This also allows water to overfill your top tank.
The problem is usually in the fill tank. If the water level in the tank is dropping, check the flapper. If it's not sealing the flush valve opening, water from the tank will leak into the bowl, and the toilet will run constantly to keep the tank filled. Replace the flapper as needed.
If the water continues running until it flows into the overflow tube, regardless of the float level, then the issue is probably caused by a faulty fill valve. However, if the water is continuously running but not flowing into the overflow tube, then the flush valve might be the problem.
Sometimes, especially in older homes, the water shut off valve can be difficult to access or operate. In these cases, spraying WD-40 on the valve may help make it easier to turn off. Be sure to test the valve afterward to make sure it is fully closed.
No need to worry, a running toilet is usually a simple fix. The three most common causes are a broken or dirty flapper, too long or too short of a chain between the flush lever and the flapper or a float that is out of position.
Check the flapper
If the flapper chain is tangled or misaligned, it won't close properly, causing water to continue running into the toilet and the cistern to continue refilling.
Replace Your Shut-Off Valve At Least Every Ten Years
Fixture shut-off valves deteriorate over time. They should get replaced every ten years or whenever you replace a sink, toilet, or faucet, whatever comes first.
Toilet Parts
The toilet's flush valve will typically last 6-7 years. A toilet fill valve might only last four to five years. In general, you should expect to replace the parts in your toilet every so often while keeping the actual bowl in place.
Die the water in your toilet tank with 2-5 drops of food colouring. Wait for 10-30 minutes. Inspect toilet bowl for any sign of died water. If you find died water in your toilet bowl, your flush valve is leaking.
There are really only two main toilet tank parts: The toilet flush valve, which lets water gush into the bowl during the flush, and the fill valve, which lets water refill the tank after the flush.
Cleaning your fill valve
Once a year, or if the fill valve will not turn off, remove the cap assembly and inspect for debris in the valve body seat. Use an inverted coffee cup over the uncapped valve body and turn the water on all the way for three or four seconds then turn off. Do this twice to rinse out the valve.
If you look inside the tank of your toilet and see that the fill valve is leaking or spraying water from either the top, or leaking water from the bottom of the tank, chances are good it needs to be replaced before it wastes you any more water or causes any damage to your home.
Critical Level Mark (Indicated with the letters CL on the fill valve) of the fill valve must be 1 inch above the top of the overflow pipe. When the fill valve height is set correctly it is possible for the top of the valve to be positioned above the rim of the tank.
If the rubber of the flapper is old and cracked, it will be unable to seal the flow of water into the tank. In this case, you'll need to replace the flapper. Or, you can replace the entire flush valve/flapper assembly with a convenient repair kit.