How do I repair a leaking toilet tank?
Make sure the gasket that the flapper rests on is perfectly clean.  Any grit or sediment on it can prevent the flapper from making a good seal.
Check the chain that runs from the handle assembly to the flapper.  If it's taut when the flapper is down, it may be holding the flapper slightly away from the gasket, again preventing a good seal.


If that doesn't work, it may be time to replace the whole flush assembly