Pop-up stoppers on bathroom sink drains are simple but ingenious devices. When you pull a rod on the faucet, the stopper drops in place like magic. Over time the stopper's seal can loosen, allowing water to drain out even when the stopper is in place. When this happens, a minor adjustment is all that's needed. You don't need to call a professional plumber or replace your entire sink--a flashlight, pliers and a new O-ring are all you require.
Inspect the rubber O-ring on the stopper. If it is worn, cracked or missing, replace it. A new O-ring can simply stretch over the top of the stopper and snap in place. Fill the sink and test the seal. If this does not solve the problem, move on to the next step.
Familiarise yourself with the plumbing under the sink, using the flashlight. There are four important parts. The stopper rod is what you pull up and down to operate the stopper. The clevis is a metal strap hanging down from the stopper rod. The pivot rod extends from the clevis into the drain pipe. The pivot nut holds the pivot rod in place on the back of the drain pipe.
Tighten the pivot nut under the sink if it is loose. Lack of pressure will not allow the stopper to lock into any one position. Do not over-tighten the nut or you could damage the drain. Fill the sink and test the seal. If this does not solve the problem, move on to the next step.
Loosen the screw that holds the clevis to the stopper rod with a pair of pliers.
Slide the clevis up the stopper rod slightly and re-tighten the screw.
Test the seal. If the stopper still allows water to drain, move the clevis higher on the stopper rod.