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I have two kitchen sinks, a P-Trap is indeed located under the floor. The problem I'm experiencing is that with heavy water flow (when open hot or cold water tap at full rate), the leak starts at "red rectangle" position. The strainer pipe fits exactly in the black plastic pipe, so there is no issue with size

I suspect there might be a blockage down the pipe, and as water goes back it leaks, but why would it not flow back in the sink then? When strain pipe enters the PVC pipe, is it supposed to have some plumber's putty/etc. applied?

I would be glad to have some advice on this. Thanks.

UPDATE As was suggested by Michael Karas, the solution is to install a compression washer on the tail pipe fitting in the black pipe.

enter image description here

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    Could PTrap be under the floor? – Michael Karas Dec 23 '17 at 16:56
  • @MichaelKaras, I updated my question. – Mark Dec 23 '17 at 20:33
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If you see a leak as you describe then there is a bad seal in the slip joint between the black ABS plastic fitting and the tail pipe that slides inside the black pipe. The seal normally consists of a rubber, silicone or neoprene washer that fits over the tail piece and under the plastic compression nut as shown here:

enter image description here

In normal installation the compression nut slides onto the tail piece then the compression washer and then the tail piece is inserted into the plastic pipe. Once the slip fit length is adjusted the compression nut is slid down and threaded to the top of the ABS fitting.

Old compression washers can get hard, dry and crack. When this happens the seal is broken allowing for a leak to occur. Your probable best fix is to replace the compression washers with new flexible ones. Consider replacing the seals on both sides at the same time and the seal rings where the tail pieces attach to the sink drainer baskets. (Note that these upper seals are a different style/size of washer). When the piping assembly is apart clean all the surfaces of the seal areas so that there is no grit, grime or corrosion that could compromise the seal.

And...No. Plumbers putty is not a suitable product to fix this type of leak.

  • +1 exactly right. I would add that the washer used at the bottom of the strainer is indeed a totally different type than those used at the slip-joint fittings. – Jimmy Fix-it Dec 23 '17 at 22:05
  • @JimmyFix-it - updated answer based upon your comment. – Michael Karas Dec 23 '17 at 22:14
  • The fact that the OP mentioned the fitment of the pipe makes me wonder if there's a compressing ring in there at all. Plus one. – isherwood Dec 23 '17 at 23:22
  • @MichaelKaras, thank you, that was exactly the root cause. – Mark Dec 25 '17 at 2:21

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