“Inside the average house, even properly functioning toilets rank as the number one water users–standard toilets use 3½ to 7 or more gallons per flush. But if your toilet ‘runs’, it could waste as much as 200 more gallons each day. Even if you can’t see or hear any water running, take a few minutes to check your toilet to be sure. All you need to do is lift the tank lid and add several drops of colored dye (blue food coloring will work or you can ask your water company for dye test tablets). Then, wait a few minutes to see if the dye flows into the toilet bowl. If it does, either the float valve inside the ball cock assembly isn’t shutting off or the tank ball (or flapper) is worn and not seating properly.
To find the exact problem, first lift up on the float ball to see if the water flow stops; you should be able to hear the change. If the flow stops, try bending the float arm down slightly so the water level will shut off the valve sooner with more force. If this works, simply replace the toilet tank’s lid.
If another dye check reveals that water still flows between the tank and bowl, turn off the water shutoff valve behind the tank, then flush the toilet and use a sponge to completely empty the tank. If the ball or flap-type valve at the tank bottom is rough, remove it and take it to the plumbing shop for an exact replacement.
Also, check to see if the valve seat is rough, scaled or corroded. If it is, dry it with a cloth and use emery paper to smooth it. With the new ball or flap installed, turn the water shutoff valve back on and repeat the dye check.
If these remedies don’t help, then the ball cock assembly’s washers or valves are probably worn or faulty. You can replace individual parts or the entire ball cock assembly. Many packages have full instructions.”