The washers inside the toilet tank that provide a seal to prevent leaks deteriorate over time, and also as a result of chemicals that may be used in the tank water. You will notice this leak especially after you flush, with water on the floor underneath the tank behind the bowl. Stop the leak by removing the tank-to-bowl bolts and washers, along with the rubber gasket and washer underneath the tank, and installing new fibre washers that protect against leaks.
Turn off the water to the toilet by twisting the wall shut-off valve completely to the right. Lift and remove the tank lid, and temporarily place it out of the way. Flush the toilet and hold the handle down. Watch until all the water drains from the tank. Wipe up any remaining tank water with a towel.
Unscrew and disconnect the inlet hose from the bottom of the tank, using an adjustable wrench. Spread newspaper on the floor near the toilet. Insert a slotted screwdriver in the slotted head of the tank-to-bowl bolts, while unscrewing the nuts underneath by hand. Use an adjustable wrench on the nuts if you are unable to make them budge by hand.
Lift and remove the tank from the bowl. Carefully lay the tank on its back on top of the newspaper. Remove the tank-to-bowl bolts, along with the washers, from the bottom inside the tank. Remove the rubber gasket that is located underneath the toilet and covers the large spud nut of the flush valve assembly. Unscrew the spud nut, using a pair of slip-joint pliers. Pull the flush valve assembly out of the tank opening and slide the washer off the assembly's tailpiece.
Look on the top of the toilet bowl, in the rear of the seat, to find the printed name of the toilet manufacturer. Write down this information on a slip of paper so you will remember it. Take the information with you to a home improvement or hardware store to make certain that you obtain the proper-sized replacement gasket. Obtain a tank-to-bowl bolt and gasket kit that contains fibre washers.
Slide the new, large fibre washer up the bottom of the flush valve assembly. Insert the flush valve in the tank opening, with the overflow pipe positioned on the side of the fill valve. Tighten the spud nut onto the tailpiece of the flush valve assembly from underneath the tank. Tighten the nut with the slip-joint pliers. Place the rubber gasket over the spud nut.
Slide a metal washer and then a fibre washer onto each bolt. Insert the bolts in the two holes inside the tank on the bottom. Make certain that the fibre washers, not the metal ones, are up against the tank bottom. These fibre washers provide the watertight seal.
Lift the tank and attach it on top of the bowl. Insert the bolts through the corresponding holes in the rear of the bowl. Steady the bolt heads in the tank with a slotted screwdriver while tightening the nuts underneath by hand.
Attach the inlet hose to the bottom of the tank, and place the lid on the tank. Slowly turn the shut-off valve counterclockwise. Check the toilet for any leaks.