Repairing a Mansfield flush valve may end up being more than just a seal repair. If you find that you need to replace the valve, you will first have to remove the Mansfield flush valve. Removing the flush valve will require removal of the toilet tank. The retaining nut is between the toilet bowl flange and the toilet tank. Once you have the valve out of the tank, take the valve to a hardware store for a replacement.
Turn the water supply off to the toilet tank at the water cutoff valve. The cutoff valve will be on the water supply line behind the toilet. Remove the toilet tank lid and set the lid on a flat surface out of the way.
Flush the toilet and hold the toilet handle until most of the water is gone from the tank. Use a few towels and a bucket to soak up the remaining water from the tank.
Disconnect the water supply line from the fill valve under the bottom of the toilet tank. Use a pair of channel lock pliers to unscrew the the water line from the fill valve.
Locate the two bolts that secure the toilet tank to the toilet bowl. Use a socket wrench to remove the nuts from the securing bolts. If the bolts spin when turning the nuts, hold the bolt heads with a flat-head screwdriver inside the toilet tank.
Lift the toilet tank out off the toilet bowl. The tank has some weight so handle the tank appropriately. Turn the tank upside down and place the tank on a flat surface.
Pull the big rubber washer off the bottom of the toilet tank to reveal the securing nut for the flush valve.
Remove the flush valve securing nut with a pipe wrench by turning the nut counterclockwise. Do not bang the porcelain tank with the pipe wrench.
Turn the tank on its side and pull the flush valve out of the toilet tank.