Tips for Identifying Toilet Leaks
The first thing to check is the water level inside the toilet tank. Every toilet tank has an overflow tube. If the water level is too high or the ballcock is out of adjustment, water will constantly spill over into the tube, which wastes water right down the drain.
There are two solutions to this problem. First, if the ballcock if fairly new, a little adjustment is all it may need. For a rod and ball float, find the adjustment screw on top and turn it clockwise to lower the water level. For a cup float style, find the locking slide that operates the float. Simply slide it down about a half inch. Remember, the proper water level for a toilet is about a half inch below the rim of the overflow tube.
For an older ballcock that won't adjust properly, simply replace the entire ballcock. Just shut off the water, drain the tank and disconnect the supply line. Next, remove the locking nut under the tank and pull out the old ballcock. To install the new one, just do the same steps in reverse.
To check for other leaks in the toilet, place a few drops of food coloring in the tank. After a minute or so, if there is coloring leaking into the bowl that means the flush valve and/or flapper is leaking.
There are two common flapper problems. The first one is the handle and chain tension. If the chain is too tight, it'll keep the flapper in a slightly open position letting water leak out of the tank. The solution is to add some slack to the chain; about a half inch of slack is recommended. Don't make it too loose or the handle won't be able to pull the flapper up all the way to flush the toilet.
The second flapper problem is a bad or broken flapper, which will constantly leak causing the water level in the tank to keep dropping. The flush valve will do its job to keep the tank full making a sound like the toilet is constantly flushing. Replacing the flapper will remedy this problem.