Likely cause: a worn or damaged stopper seal

Why this happens

Lowe's says to inspect the rubber seal on the bottom of the stopper head and to replace the stopper if the seal is cracked or brittle. Danco lists a cracked or dirty seal among the reasons a pop-up stopper needs replacing. Delta explains that this rubber seal is what the water presses against the flange, so a damaged one lets the sink drain down.

How to fix it

1. If the old stopper is locked in by the pivot rod, you will need to open the drain. If a chemical drain cleaner has gone down this sink, stop here and call a plumber instead: Family Handyman says leftover caustic chemicals are a danger to anyone who takes the trap apart. Otherwise let the sink empty, then close both stops under the sink so the faucet cannot be run, and set a pan or bucket under the drain pipes, as This Old House advises. If a stop will not turn, do not force it. Leave the faucet off instead.
2. Once the sink is empty and the step above is done, unscrew the nut that holds the rod in the back of the drain and slide the rod out until the stopper lifts free. Danco says to save any washers inside the nut.
3. Keep the old stopper so you can match it.
4. Set the new stopper in the drain the way its own instructions show. On Danco's universal stoppers, for example, the tab faces the pivot rod.
5. Slide the rod back in so it engages the stopper, following the instructions packed with the new stopper. Pfister says to wiggle the rod up and down to confirm the stopper moves with it.
6. Thread the nut on with its washers. Danco's instructions say not to overtighten it.
7. Open the stops, pull the lift rod up and fill the sink. If the new stopper does not drop fully, adjust the linkage.

When to stop. If a new stopper drops fully and the sink still empties, the leak is likely at the drain flange. Stop there and see the drain seal result, or call a plumber.

If you need the part

Match the old stopper before you order. Bob Vila says to check the product information to make sure a replacement stopper is compatible with your sink, and Pfister says your faucet model's parts diagram gives the correct part number.

Start this troubleshooter over

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