Signs

• A leaking fill valve will cause the toilet to constantly run, with an unyielding hiss as water constantly flows into the tank. A tank that shuts on and off, running intermittently, could signal a fill valve that opens and shuts again and again after it has already filled the tank.
  
  

Identification

• Remove the tank lid to see if the level of water in the tank is rising so high that it runs into the white overflow pipe that stands up in the tank. Flush the toilet, and you may be able to easily identify water spraying from the leak in the fill valve.
  
  

Problem Repair

• Replacing the fill valve is often easier than attempting to fix it. Turn off the water to the toilet, flush the toilet, then suck the remaining tank water out with a wet/dry vacuum before removing the water supply hose. After replacing the leaking fill valve, reinstall the supply line, and check for any leaks as you turn on the water and flush the toilet.