When we were children, plugging a lavatory sink--more commonly known as a washbasin--involved placing a rubber stopper into a drain. The stopper was tethered to the lavatory faucet by a small metal beaded chain so that it would be there for the next person.

The rubber stopper was not unique to our lavatory. The same configuration existed at our bathtub with one minor difference--the rubber plug was slightly larger for the tub drain. In either case, it wasn't very glamorous, but it did the trick. However, over time, the rubber stopper would become brittle, crack and invariably leak. And the chain would usually be the source of rust.

It wasn't until a visit to a friend's home in the newer part of town that we discovered a leak-free, rustproof alternative. Our rubber plug had become obsolete and had been replaced by a sleek, modern marvel that had neither a chain nor a rubber plug. It consisted of a small metal rod that operated a stopper that remained in the drain. When the rod was raised, the stopper would seat in the drain; when the rod was lowered, the stopper would ``pop up.'' We thus had our first experience with a pop-up drain assembly.

Although a new lavatory faucet can still be purchased without the pop-up assembly, it is now considered standard equipment on most brands. Several variations of the lavatory pop-up are used for bathtub drains.

To install a new faucet with a pop-up drain assembly, the existing drain and tailpiece must be removed as well as the existing faucet. Begin by turning off the hot-water and cold-water supply valves. Use a small wrench to disconnect the water-supply lines from the faucet and remove the faucet from the sink. Next, with a bucket placed directly below the drain, disassemble the P trap and remove the tailpiece and drain from the sink.

Before installing the new drain assembly, install the new faucet and reconnect the water-supply lines. If the water-supply lines display any hint of corrosion or wear, replace them with new reinforced water-supply lines. Use Teflon tape at all threaded connections to prevent leaks. Also, thoroughly clean the mouth of the drain in the sink to ensure that the new drain will seat properly. Remove existing putty, using a plastic putty knife (to avoid damaging the sink) and wipe clean with a dry cloth.

For the best possible installation, place the new drain flange into a fresh bead of plumber's putty. Secure the drain flange by installing and tightening the lock nut at the underside of the sink. Make certain to install the supplied washer between the sink and lock nut. Next, screw the drain T into the drain. Then screw the tailpiece into the drain T. Often, the tailpiece is part of the drain T. The hole in the side of the drain T should face the rear of the sink in direct line with the lift rod.

Feed the pivot rod into the drain T, and tighten the retaining nut. The pivot rod has a round ball that pivots in as the lift rod is moved up and down. Insert the lift rod down through the faucet body (or through the top of the sink, depending upon the configuration) and fasten its lower end to the clevis using the clevis screw. The clevis is a flat metal rod with several holes. It connects the lift rod to the pivot rod. Feed the pivot rod into one of the clevis holes and make it fast with the spring clip. Test the mechanism by raising and lowering the lift rod and, if necessary, reset the lift rod and clevis.

Complete the job by reinstalling the P-trap assembly.

The secret to a leak-free drain and trap installation is to avoid over-tightening the fittings. Hand-tightening is usually all that is needed to make leak-free connections.