How to Repair a Running Toilet 1. Adjust the Float. The float arm controls the water level in your tank. If it’s set too high, water will keep running into the overflow tube.

1. The flapper. The flapper, which creates a seal in the bottom of your toilet tank and then lets water flow into the bowl when you flush, is one common cause of running toilets.

Low water in a tank can be caused by float balls, which stop water from coming in once it reaches the right height. ... The fill valve is what controls the amount of water that goes into the tank.

If the fill valve, float, or flapper (the seal that moves when you operate the handle, letting water out and into the bowl) are malfunctioning, water will leave the tank, drop the water level, and ...