How to repair a Mansfield toilet step 8

This is the old Mansfield valve seal, notice that is is beveled, this beveling happens when the seal gets old, or if it is exposed to drop in bleach tablets, and other products that people put in to the toilet tank, to help keep the toilet clean, unfortunately these things cause all the rubber parts in the toilet tank to fail.
Hot water in the toilet tank will also cause the toilet valve seal to get soft and fail. The beveling of the seal is what causes the flush valve to be so hard to lift, it is designed to be flat.

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Mansfield old worn disk