I have just installed vessel lavatories in new bathroom. There is no problem with draining, but despite tightening I have leaks from the trap compression nuts and where the tailpiece goes into the extension. Water seems to flow down while faucet is running but shortly thereafter, it appears at the top of the tailpiece connection and at both connections of the trap. Everything appears to be lined up properly, but can't stop the leaks.

Nug the first things I would do are to make sure that the trap compression nuts have the proper plastic cone gaskets installed. All but the center nut of the trap should have cone gaskets and the thick part of the cone should be against the compression nut.

Also, if the gaskets are in place I might recommend trying some teflon paste on the threads.

On your tailpiece connecting to the extension, I would definitely recommend teflon paste on the mail threads of the extension. Hand tight plus a quarter turn with some channel locks should make you leak free on that connection.

Nug, I have trained many to be plumbers, and this is one area I specifically address. Let's look at the drain in it's entirity. I will describe the common lavatory [lav] waste. First you have the top part, usually the chrome piece that you see. It is screwed to the drain piece that goes through the lav. I use thread sealant [liquid] on the threads connecting that. Then there is the rubber washer usually slightly cone shaped that seals the bottom of the lav with a slip washer and nut below that. This is usually where the leak is. The threads on that stalk are straight cut and usually deep. If you tighten the bottom nut really really tight you can pop the top chrome piece off or crack the lav. If it is not tight it will leak. It leaks on those deep threads. I use thread compound also on those threads because water will walk down those deep cut threads inside the rubber seal. Using the compound on those threads you don't have to overtighten the bottom nut. Then usually there is a tailpiece that screws into the drain stalk. I use thread compound on those threads too. In the installation of the trap, remember the beveled part of the washer is the seal. It goes down. The flat part of the trap washer backs up to the nut. Usually this nut can be hand tight if you have strong hands. You also could have a leak where the plunger rod goes into the back of the drain.
Basically though, to find the leak, get down there and do a visual inspection of the whole bottom of the lav while water is running. I have had guys that will feel for a leak and never find it, but it was there.

then usually there is a tailpiece that screws into the drain stalk. i use thread compound on those threads too. in the installation of the trap, remember the beveled part of the washer is the seal. it goes down. the flat part of the trap washer backs up to the nut. usually this nut can be hand tight if you have strong hands.



The chrome piece where it attaches to the lav is fine, but the bottom of the tailpiece that goes into the drain stalk has no threads. The leak is here between the bottom of the chrome tailpiece as it goes into the drain stalk(?). The tailpiece is 1 1/4 and the drain stalk is 1 1/2. Is that the problem? Should I be using a compression type sleeve that matches both ends?

Most pvc master traps come with a 1 1/2" x 1 1/4" slip joint washer that is snug on the 1 1/4" tailpiece and covers the 1 1/2" nut for the 1 1/2" trap.