The pop-up sink stopper is the visible component of the same assembly as the horizontal pivot rod. The pivot rod is hidden beneath the sink, acting to lower the stopper when you pull the lift rod up and to raise the stopper when you push the lift rod down. The lift rod connects to a clevis, or connecting bar, beneath the sink, which connects to the pivot rod. One end of the pivot rod fits through a drain opening, into the hole in the stopper bottom. Replacing the stopper and pivot rod is a simple repair, but to complete it, you must venture beneath the sink.
Push down on the lift rod to raise the stopper in the sink drain. Lay the screwdriver beneath the top of the stopper, going across the drain opening, to block the stopper from closing.
Loosen the retaining nut, which fits the end of the horizontal pivot rod snugly into the back of the sink drain, underneath the sink. Use your hand or, if necessary, use an adjustable wrench to loosen the nut. Pull the end of the pivot rod from the opening, disengaging the pivot rod from the stopper. Slide the spring clip and clevis from the other end of the pivot rod and remove the pivot rod.
Lift the stopper from the sink drain. Pry the stopper up, using the screwdriver, if the stopper has dropped back down into the drain opening. Hold the replacement stopper over the drain, making certain that the hole in the bottom of the stopper faces the back of the drain. Carefully drop the stopper straight down into the drain.
Position the replacement horizontal pivot rod so that the end of the rod with the white pivot ball on it faces the sink drain. Slide one end of the spring clip off the opposite end of the pivot rod, and insert the end of the rod through a hole in the clevis. Slide the end of the spring clip back onto the pivot rod, bracketing the clevis.
Shine the flashlight into the drain opening to help you locate the bottom hole of the stopper inside the drain. Insert the end of the pivot rod into the drain, making certain that it goes through the hole in the stopper. Tighten the retaining nut, enclosing the plastic gasket, pivot ball and rubber washer, as you tighten the nut onto the drain.
Lift up and push down the lift rod to make certain that the new stopper and pivot rod work properly. Loosen the set screw on the clevis and adjust the clevis up or down to allow the pivot rod to open and close the stopper properly. Tighten the set screw.