In this video, This Old House plumbing and heating contractor Richard Trethewey unclogs a stubborn sink sink drain.
1. Use pliers to remove the threaded plug from the underside of the sink trap. (If the trap doesn't have a drain plug, remove the entire trap.)
2. With the plug removed, check to see if water drains freely through the trap. If not, remove the trap and clean it out.
3. Disconnect the elbow from the garbage disposer and check for clogs.
4. Remove the vertical tailpiece that extends down from the sink drain.
5. Check for clogs in the baffled T-fitting that's attached to the bottom end of the tailpiece.
6. Replace the old plastic baffled T-fitting with a full-sized brass T-fitting that has no baffle.
7. Tighten all the plumbing connections with pliers.
8. Replace the threaded drain plug in the bottom of the sink trap.
9. Lay some paper towels across the floor of the sink cabinet.
10. Fill the sink with water and then pull the drain.
11. Check to be sure the water drains rapidly and that none of the plumbing connections leaks; drips will appear on the paper towels.