A sink's pop-up stopper is comprised of a stopper and a linkage system that controls the position of the sink drain plug. After awhile the pop-up stopper may not move anymore when you push on the lift rod, or the stopper does not close all the way and water escapes down the drain. Occasionally sink stoppers need adjustment, and a wrench or a screwdriver can usually fix the problem.
Locate the pop-up stopper pivot nut that is on the back of the sink drain pipe if the stopper will not stay closed. Sometimes water may seep through and form a puddle under the sink. Tighten the nut by turning a wrench counterclockwise, which will help the stopper to hold its position either open or closed. Be careful not to over tighten or you won't have the ability to open the lever on the pop-up stopper at all.
Adjust the pop-up stopper linkage assembly if the above step does not solve the problem and the stopper is still not staying closed. Peer under the sink and look up. Locate a horizontal metal arm that extends from the pivot nut to a metal or plastic linkage assembly that is attached to the lift handle rod. On some sinks a screw holds the lift handle rod to the horizontal arm, and in other cases a U-shaped piece of metal falls down the lever rod.
Tighten the screw with a flat screwdriver if this is what you have. Push the U-shaped metal piece up or down, depending which way it moved. Test the stopper and readjust accordingly.
Loosen the pivot nut on the pop-up stopper if you find the lift rod on the stopper is difficult to move up and down. Turn the wrench counterclockwise to loosen the nut.
Re-attach the horizontal metal arm to the pop-up stopper if it becomes disconnected. Loosen the pivot nut with a wrench so you can move the metal arm in and out of the drain pipe. Align the metal arm hole in the base of the pop-up stopper assembly. Once you have the metal arm aligned, tighten the pivot nut with a wrench by turning it clockwise.
If the pop-up stopper lift rod is still hard to move even after you loosen the pivot nut, place a small amount of vaseline on the pivot ball with your fingers to help it move more smoothly.