Built-in sink stoppers provide a convenient way to seal the sink's drain. When you notice that the sink's water level drops even though the stopper is supposedly closed, you need to find out what the cause of the problem truly is. Several factors can cause the stopper not to sit low enough in the drain, or the stopper's seal not to function correctly.
Check for Clogs
Back the retaining nut from the connection on the side of the drainpipe, then pull the pivot rod out of the pipe. With the pivot rod out of the drainpipe, you are free to lift the sink's stopper out of the drain opening. Clean off any hair or soap build-up on the stopper, which can keep the stopper from lowering all the way down so that it seals the drain off. Reach into the drain opening and pull out any debris you can feel as well.
The sink's stopper has a rubber seal that attaches to the underside of the top of the stopper, keeping the water in the sink from slipping past the stopper and exiting out the drain. While you still have the stopper out of the sink's drain, use the opportunity to inspect the seal fully for signs of wearing. If you see evidence of cracking or the seal looks wavy or warped, remove the seal and slide a new one into place.
Adjust Stopper Mechanism
Replace the sink's stopper and insert the pivot rod back into the opening in the drainpipe. Thread the retaining nut back onto the opening. Adjust the height of the clevis, or the metal strap piece that the other end of the pivot rod is inserted through. You will need a screwdriver to loosen the clevis strap's retaining nut, which holds the clevis onto the end of the lift rod. The retaining screw is located on the "C" shaped portion of the clevis. Loosen the screw, but do not remove it. Push on the top of the stopper so it sits firmly in the closed position, allowing no more water to leak out of the sink. Tighten the screw again to keep the adjustment from changing.
After you have corrected the problem, you need to be vigilant to prevent the same problem from developing later. Periodically remove the sink's stopper and clean out any hair or soap scum, which will help keep the drain working properly.