Combined with a float ball that rises and falls with the water level, the ball valve directs water flow inside a toilet water reservoir. The ball valve on the water intake pipe opens when the ball float is lower than the desired water level and closes tightly, preventing water from flowing into the tank when the ball reaches the top of the water line. If your toilet has a ball valve and runs non-stop, it could be the float or ball valve. Both are easy to fix and you can replace them in a couple of hours.
Close the water shut-off valve. Look for it connected to the water pipe behind and just to the left of the back of the toilet. If there is no valve at the toilet, or you can't turn it by hand, shut off the water supply to the house until repairs are finished.
Flush the toilet to be sure the water if completely off. No water should flow back into the reservoir tank once flushed. If water does continue to flow into the tank, you must use the whole house water shut-off valve instead of the toilet water shut- off and put the shut-off valve on your list of things to replace in the future.
Remove the water line from the bottom of the toilet using an adjustable wrench or pliers. Remove the lid from the toilet reservoir by lifting it off and setting it aside.
Disconnect the nuts holding the ball valve and water supply pipe to the reservoir using a pliers. Check all washers inside the valves for cracking, tearing or other reasons to replace them.
Fit new ball valve with the existing, or new, washers and place it through the bottom of the reservoir. Wrap the threads of the valve with plumber's tape and attach it to the water pipe.
Replace and tighten all nuts to secure the ball valve and the water pipe in place. Attach the float ball arm to the valve.
Open the water supply either at the water shut-off valve behind the toilet, or at the whole house water shut-off valve. Check the water level to be sure the tank fills properly. Adjust the ball float if the water levels are too high, or low. Put the lid on the reservoir.