The three most common causes of a leaking overflow pipe are increased water pressure, an overused washer or a faulty float valve. It is important to fix a leaking overflow pipe immediately, as it can lead to structural damage to a house and mold formation.
If there is very high water pressure in the tank, it can continually force water up and out of the overflow pipe. To fix this issue, partially tighten the inlet valve of the tank to lower the water pressure. The inlet valve controls the flow of water into the tank. It can commonly be found either in the kitchen or near the tank.
If the washer of the tank is over worn, it can lead to a leaking overflow pipe. The washer of the float valve often becomes worn out because it is constantly immersed in water and, as a result, the sediments in the water. Simply remove the washer and replace it. Make sure to close the inlet valve before removing the washer to keep the tank from overflowing.
If it is determined that the washer of the float valve is in working condition, then the float valve itself could be the cause of a leaking overflow pipe. See if there is water inside the ball of the float valve. If so, replace it. Again, make sure to turn off the inlet valve first.