I have an issue with my one toilet that is driving me crazy!!!! It will run by itself sometimes for about 4 seconds which indicated I have a leak somewhere inside. 90% of the time the flapper valve is the problem. I changed it, and made sure the chain was not caught on anything and still water gets into the bowl. I then tried a test I learned from a plumbing friend of mine: Mark the back of the tank at the water line with a pencil. Then turn the water supply off. Wait 30 minutes. If water level stays at the pencil mark, the leak is occurring at the refill valve. It the water level falls below the pencil mark, the leak is the flush valve. When I turn the water supply off, the water completely empties from the tank. Many people told me the flush valve rarely goes bad. Any suggestions??