In this video, This Old House plumbing and heating expert Richard Trethewey shows how to replace a leaky, glued-together kitchen-sink trap
1 Remove the drain plug from the bottom of the sink trap with pliers; use a foil baking pan to catch the water that drains out.
2 Use a reciprocating saw to cut out the existing PVC sink trap.
3 Loosen the compression fitting that connects the trap to the sink drain; remove the entire PVC assembly.
4 Use a special PVC cutterhead in a drill to remove the remaining piece of pipe from inside of the drainpipe in the wall.
5 Glue a short section of PVC pipe to a reducing coupling. Be sure to apply both PVC primer and PVC cement to both parts before pressing and holding them together.
6 Next, use PVC primer and cement to glue the reducing coupling to the drainpipe coming from the wall.
7 Dry-assemble (no glue) a new sink trap from 1½-inch PVC pipe.
8 Test fit assembly under the sink to ensure it fits properly, then glue together the PVC parts.
9 Use pliers to tighten the compression fitting that connects the trap to the sink.
10 Fill the sink with water, then slip a paper towel inside the cabinet, directly beneath the new trap.
11 Pull out the plug from the sink drain, allowing all the water to rush out.
12 Check the paper towel for any wet spots, which would indicate a leak.