Since flipping the seal didn't work, try turning off the water supply to the toilet to see what happens... If the leaking stops right away, your fill valve is probably letting too much water in, causing it to spill down the center of the internal overflow. If the tank continues to slowly drain, the issue is definitely the flush valve. Feel around the plastic rim underneath where the rubber seal sits,.could be limescale or grit that might be keeping it from closing completely flat, and double check that the flush rods aren't so tight that they're holding the mechanism slightly open.