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I've just replaced the trap on a sink drain, and I can't get the replacement to stop leaking. The leak is coming from the where the trap connects to the wall stub (circled in the picture).

There is no washer here; it looks like this is supposed to be a pressure fit with the outside nut forcing the sides of the conical end on the bottom piece against the sides of the top piece. It's a great theory, but it doesn't seem to be working.

I've tried reeeeeeally tightening this joint, and while that seems to have reduced the leak it hasn't stopped it. Since this isn't really a screw joint, there's no point in something like plumbers tape.

Any thoughts on how I can stop this from leaking?

Drain assembly with leaky trap joint circles]1

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    Is there a reason you constructed that out of glued pipe & fittings instead of using a a standard P-Trap + Extension? That piece you have at the top end pointing up & connected to the chrome pipe is what you should have coming horizontally out of the wall. – brhans Dec 3 '18 at 1:49
  • Carefully check the union nut on that glued-on trap, it is not uncommon to find them with a barely visible hairline crack causing leak. – Jimmy Fix-it Dec 3 '18 at 1:56
  • It's possible that the flared part (red circle) is actually at a angle and is messing up the seal.I would loosen that connection and the connection where the drain connects to the sink pipe and then tighten the flared pipe nut first and then the nut at the sink to ensure the flared fitting has the best connection possible. You can put some teflon tap around the flared part to see if it helps as well. – HazardousGlitch Dec 3 '18 at 19:52
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I'd say take it all out and start over. Avoid RTV and flexible fittings. Put a standard tailpiece onto the sink, a starndard trap like brhans mentioned, and a slip nut on the pipe going into the wall. Then it will easier to take apart and clean. Or take apart in case you drop something value able down there. Google image search "standard p trap setup". Or just look under somebodies sink.

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If you can't get that joint to stop leaking try this; remove the trap pieces and drain the water, put everything back together and add a small amount of RTV silicone to the joint and let stand overnight. I had one of these that just would not stop leaking due to (?). This worked for me.

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The next joint toward the wall is usually a slip joint secured by a plastic seal and a nut. I think that this allows in-and-out and rotational adjustment which is necessary to properly position the two parts of the trap joint to achieve a seal.

It looks like what should be a slip joint is instead glued and fixed.

In order for the joint to be a slip joint the tubing used for under cabinet drain plumbing is smaller in diameter and thinner walled than the fixed drains in the walls. It looks like you have used heavy wall Schedule 40 tubing for the undersink drains.

Still it is possible you could get the leak to stop if you loosen the slip joint under the sink, then tighten the union of the trap, then re-tighten the slip joint.

If this doesn't work, this would mean you need a more flexible coupling between the larger tubing and the tailpiece (the metal tube under the sink). There are rubber reducing couplers that might work. You'd cut off the white plastic reducer and use a flexible rubber reducing splice which seals with band clamps.

Or you could cut the tail piece and use a flexible coupling to make a more flexible tail piece.

Another possibility is to use a flexible tailpiece.

But in the end you might have to remove all the too large tubing, put a proper slip joint on the stub in the wall and replumb with the correct (1-1/2 inch?) under-sink drain tubing.

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    google rubber trap and look at the pictures – jsotola Dec 3 '18 at 1:11

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