In this video, This Old House plumbing and heating expert Richard Trethewey installs a new flush valve.
1. Close the shutoff valve behind the toilet to stop the flow of water.
2. Remove the lid from the toilet tank. Flush the toilet and hold down the flush lever to drain as much water from the tank as possible.
3. Use a large sponge or cotton cloth to sop up the remaining water from inside the tank.
4. Disconnect the water-supply line from the underside of the toilet tank.
5. Use a slotted screwdriver and open-end wrench to remove the two bolts that hold the toilet tank to the bowl.
6. Lift the tank off the bowl and use pliers to remove the large nut on the underside of the tank. Remove the old flush valve from the tank.
7. Install a new flush valve to the tank; securely tighten the mounting nut with pliers.
8. Attach two new brass bolts to the tank.
9. Place the rubber spud gasket onto the bottom of the flush valve, then set the tank onto the toilet bowl.
10. Put washers and hex nuts onto the brass bolts, then alternately tighten the nuts to secure the tank to the bowl. Be careful not to overtighten the nuts or you'll crack the toilet.
11. Reconnect the water-supply line to the underside of the tank.
12. Adjust the flapper chain to the correct length.
13. Turn the water back on and flush the toilet to ensure it's operating properly and quietly. Adjust the flapper and overflow tube, if necessary.