You may have a problem with the seat. The part the flapper covers. I have had this problem. As best I can recall I think I purchased something that fit into the seat and sealed it then the flapper on top of that. It was sold as one piece. Next time I had the problem, different tolit, I unbolted the tank and replaced the seal between the tolit and the tank. Look for two screws in the bottom. I would go for this solution if you have a two piece tolit.
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