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There is a slow leak in the cabinet under my bathroom sink.

It seems to be coming way up at the top of the cabinet, from a black plastic connector. (See photo.)

I tightened the black plastic connector and that helped but it still drips every 20 seconds or so. I do not think the steel connection is leaking.

How might I go about fixing this leak?

Underside of sink top of sink

3 Answers 3

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The black plastic faucet nut is securing the faucet to the sink. If the water is coming from the porcelain/black nut interface then you could caulk the outside interface of the faucet to the sink on the sink deck. It looks from your 2nd picture that there is a rubber gasket between the faucet and the porcelain sink so you might just need to crank the black nuts tighter to ensure no water from the sink deck migrates under the faucet gasket into the sink cabinet.

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  • I do not think the steel connection is leaking. If that's true I think that the answer by @Fresh Codemonger is likely to be the correct one since the only other place it can come from is standing water around the faucet base. That was going to be my answer as well. If the gasket under the faucet has deteriorated you might try a bead of caulk around the faucet base.
    – HoneyDo
    Oct 26, 2021 at 3:22
  • Prior to caulking around the base of the faucet, removing it and putting a bead of plumber's putty under there might just do the trick and be more removable.
    – FreeMan
    Oct 26, 2021 at 12:17
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Check that the faucet valve stems are not leaking, that's the most likely cause of a steady drip from above that plastic nut.

Replacing the O-rings in a faucet valve is a fairly easy task.

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The black plastic piece holds the faucet fixture to the porcelin sink. It's primarily a "structural" connection and not directly related to containing water.

The compression fitting below that, between the steel water supply line and the brass fitting, is what seals the water. Inside the steel "nut" there's a rubber compression gasket that gets drawn up against the edge of the brass piece and seals the connection.

Image stolen via Google Image Search

The supply line may not be tightened enough to prevent water from leaking, or the rubber compression gasket may be damaged or mis-aligned. Most likely it isn't tightened enough. If it doesn't seem to be able to be tightened, check the gasket.

In order to tighten the nut, you may need to use a tool called a "basin wrench" or "sink install tool" to reach up and turn the fittings. Be careful not to break the black plastic ones!

Image from Family Handyman Magazine via Google Image Search

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