How to repair a leaking toilet tank

If your tank is constantly leaking water, but you've not found it anywhere, then it is nigh on certain that it's going into the toilet bowl. Tester101's tip of putting food colouring into the tank and seeing if it appears in the bowl will likely confirm this.

The issue is then to find exactly where the leak is. Either your flapper isn't closing properly, there's a leaky joint where the overflow pipe connects (just below the flapper), or the seal between the flapper apparatus and the tank isn't watertight.

I would hazard a guess at it being the seal between the flapper apparatus and the tank, probably due to either under-tightening or over-tightening when installing the new flush. Over-tightening is perhaps the more likely - it shouldn't be tightened too much or you will distort the rubber seal.

If you are having trouble locating from what area of the tank is leaking you can add a blue colored food dye to the reservoir (tank) to see if it appears in the bowl. If you see the water is turning color then concentrate your search at the tank/bowl and/or flapper section. Although 4-5 years may not seem like a lengthy amount of time, for rubber and plastic parts it can seem forever. The flapper, if 4-5 years in service, should be replaced. Also make sure the fill valve, specifically the float, is not adjusted to high. It should cut out the water flow when filling about 1 inch below the overflow pipe that the flapper is attached to. The last place to look for the offending leak is the exterior of the bottom of the tank (this would be obvious as water would appear on the floor). The rubber washers that seal the tank to the bowl can deteriorate and crack as can the large washer that the flapper closes onto. The washer on the fill valve is on the exterior and one inside. Check with a flashlight for any water trails. One last place to check is the top of the fill valve were the float (if set up as such) is attached. If possible open the cap on the valve top and look for trapped debris that prevents the valve from closing fully.