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My kitchen sink drains very slowly. From full it takes more than a minute to fully drain, and it is hard to perceive the water level dropping.

Listening in the cabinet underneath I do hear the trickling of water. I removed the S-trap to clean it, and to see whether it was blocked, but there was no significant debris on or in it, just some slime on the side.

After putting it back, I tried whether opening the aerator cap (at least, that's what I think it is, marked in red on the picture below) on the S-trap helped with the draining. It did improve the draining speed significantly, but water also leaked through the cap, so clearly it's not intended to be opened.

What else can I do to improve drainage speed?

enter image description here

JAD
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  • Where in the world are you located? Plumbing codes vary. B) that sharp kink at the top of the trap is unusual for the US and could cause issues. C) It's quite likely that there's a clog further down the line than in the sink trap - do any other drains run slowly? D) that circled item could be an AAV, and they tend to fail and need to be replaced somewhat regularly.
    • – FreeMan Feb 23 '23 at 19:00
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  • Netherlands. C) when I run the tap, the water backs up pretty much immediately, which leads me to believe that the bottle neck is pretty early in the drain. What does AAV mean?
    • – JAD Feb 23 '23 at 19:08
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    Ah, Air Admittance Valve. Yeah, it might very well be. When I opened it up, there was a rubber circle on a pin inside. How might I diagnose whether it failed? Or maybe the symptoms are an indication that it already failed? – JAD Feb 23 '23 at 19:10
  • It looks replaceable, so why not just get a new one? – Huesmann Feb 24 '23 at 13:39
  • A bad air admittance valve will not cause this problem, it will cause siphoning and gurgles in the trap. – Jasen Feb 25 '23 at 00:18