Replacing the flush valve seal on a Mansfield 160 toilet is a straightforward process: 1. Turn off the water supply to the toilet. Flush to empty the tank. 2. Disconnect and remove the old tank-to-bowl gasket. Clean any residue from the flush valve opening. 3. Apply plumber's grease around the new flush valve seal. Insert it into the flush valve opening and press into place. 4. Reconnect the water supply and test flush several times to check for leaks.