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Another potential cause of a leak is a dirty flush valve (flapper) itself. Over time, the rubber or plastic material of the flapper can accumulate grime, mold, or mineral deposits, preventing it from seating correctly on the flush valve opening. Remove the flapper from its pivots (if applicable) or lift it to thoroughly clean both its top and bottom surfaces. Again, use a sponge with some cleaner to ensure all surfaces that come into contact with the flush valve opening are spotless. A clean flapper will create a much better seal.