Take a few minutes to figure out why you have a run-on toilet, and then you can repair it. You have a run-on toilet if each flush doesn’t end with a gurgle but instead continues with a hissing sound, with water running into the toilet bowl.
To find the source of the trouble, remove the toilet tank top and place it in a safe location. Then push the flush lever and watch what happens. Don’t worry about the water in the toilet tank — it’s clean.
Run-on toilets are usually caused by a problem with the tankball, the ballcock or intake valve, or the floatball.
Sticky flush valve. If, after you flush, the water keeps running until you wiggle the flush handle up and down, the problem is probably with the linkage between the flush handle and tankball.
Sticky tankball or flapper valve. The tankball or flapper valve is sticky if isn’t falling into the drain properly.
Floatball problems. A misadjusted or damaged floatball, on the other hand, is usually the cause of water dribbling into the toilet tank, running out the overflow tube into the toilet bowl, and then going down the drain.
Ballcock problems. A ballcock that doesn’t close completely is another possible cause of a leaking toilet.
Once you've identified the cause you can start on the repairs.