If you've tried replacing the toilet flapper but the toilet still runs, the flush valve seat is probably rough or pitted. You can replace the entire flush toilet flapper valve, but it's a big job.
If it's the flapper and you're constantly changing flappers, it might be that the opening that the flapper sits on is damaged and the flapper isn't able to make a good seal. If this is the case, replace the overflow tube as the opening that the flapper sits on is integral to the overflow tube.
Sometimes when a flapper gets old and begins to fail (disintegrate) it can leave a piece behind stuck to the outflow pipe that it covers. This piece/remnant then prevents the new flapper from getting a good seal. You might want to clean out the tank and make sure there are no remnants of the old flapper stuck in there.
What it shouldn't do is let water through when it's not lifted. But rubber breaks down over time -- which is why it needs to be replaced periodically. HM recommends replacing flappers every 3-5 years regardless of whether you have a running toilet problem.
Check for: Debris: Clean any debris that may be clogging the valve. Float Adjustment: Adjust the float to ensure it's set at the correct level. If the float is too high, it can cause water to overflow into the overflow tube. Look for any signs of leaks around the connections and seals. Tighten any loose connections.
A toilet that cuts on and off by itself, or runs intermittently, has a problem that plumbers call a "phantom flush." The cause is a very slow leak from the tank into the bowl. This problem is almost certainly caused by a bad flapper or flapper seat.
If the toilet doesn't flush completely unless you hold the handle down for the entire flush cycle, it's usually because the flapper is not fully lifting away from the flush valve. This problem is caused by too much slack in the lifting chain that connects the flush lever to the flapper.
If the flapper is worn or the seat that the flapper rests on is damaged, the water level will stop just below the flapper. If the gasket that seals the flush valve into the tank is deteriorated and leaking, the water would most likely drain completely out of the tank.
Vaseline..... put a gob of Vaseline on your finger, open the valve and run it around the perimeter. Don't be stingy, use lots. Problem solved.
Step 4: Check the Flush Valve
If you've replaced the flapper and adjusted the float ball, but the toilet continues to run, the most likely cause is a leaking flush valve. Usually, such a leak is around the mouth of the flush valve where the flapper is supposed to make a good seal.
If the water in your toilet tank fills with water or runs at seemingly random times, it has a slow leak. This is wasting water and needs to be fixed. The most likely cause of this leak is a bad flapper or flush valve. A flapper is rubber and will degrade over time due to minerals and chlorine in the water.
A faulty flapper means your toilet tank will empty and fill up repeatedly. This constant filling and refilling of the tank over days or weeks can result in a lot of wasted water—and wasted money.
If you've tried replacing the toilet flapper but the toilet still runs, the flush valve seat is probably rough or pitted. You can replace the entire flush toilet flapper valve, but it's a big job.
Worn Out or Warped Flapper: Over time, the rubber material of the flapper can deteriorate, become brittle, or warp. This wear and tear can prevent the flapper from creating a proper seal, allowing water to leak continuously into the bowl.
Ghost flushing, or phantom flushing, occurs when your toilet tank refills independently without anyone pressing the handle. Water leaks from the tank into the bowl, causing the fill valve to activate periodically to refill the tank. It's similar to your toilet flushing itself, but it's just refilling.
Toilet flappers usually last for about 3 to 5 years, but the lifespan can vary depending on factors such as the quality of the flapper, the water quality, and the frequency of use.
A silent leak can be caused by a fill valve set too high.
The fill valve is the part of the toilet that is responsible for refilling the tank after being flushed. Over time, a worn or out of alignment fill valve can become a problem, cause a leak, and wear on valve's components and parts.