When you open up the cabinet under your bathroom sink and see water, chances are good that you have a leak. Part of knowing how to fix a leak in a bathroom sink is figuring out exactly where the leak is coming from. Once you know where the leak originates you can repair or replace the defective part. Bathroom sink leaks are usually caused by a bad drain pipe, loose pipe connection or leaky sink drain flange. Fill the sink with water and determine where your particular leak is located.
Shut off the water to the house.
Use a pair of channel lock pliers to loosen the slip nut that attaches the sink trap to the main drain line coming from the wall. Loosen the nut without disconnecting the sink trap. Disconnect the sink trap from the drain coming from the sink by loosening the slip nut connector. Pull down slightly on the sink trap to remove it from the sink pipe and swing the sink trap out of your way.
Use channel lock pliers to remove the slip nut connecting the sink drain pipe to the sink drain. If your sink drain has a built in stopper this will not be necessary. You will need to disconnect the stopper mechanism from the pull stem. There is a thumbscrew that connects the two. Turn the thumb screw counterclockwise to release the stopper mechanism.
Use a pipe wrench to remove the retaining nut securing the sink drain to the sink. If necessary, tap the sink drain from the bottom to break the old plumber's putty seal. Lift the sink drain out and inspect it. Clean the sink hole using a plastic putty knife to remove any plumber's putty. If the sink flange is bent or damaged, replace it. Most likely you will only need to clean the underneath of the flange to remove old cracked plumber's putty.
Roll a small ball of plumber's putty in the palm of your hands to form a thin rope. Set the rope of plumber's putty on the underside of the sink drain flange. Set the sink drain back into the sink and press it firmly in place. Attach the retaining nut to the sink drain from underneath the sink.
Wrap plumber's tape around any pipe threads. Reconnect all of the pipes in the order they were removed. Tighten down all of the slip nuts using the channel locks.
Shut off water to the house.
Use channel lock pliers to remove the slip nut connecting the sink trap to the wall drain pipe. Disconnect the drain pipe from the sink drain directly underneath the sink. Remove the drain assembly from the sink cabinet. You will have the sink trap and tail piece in one piece.
Take the old assembly with you to the hardware store. Match the new sink trap and tail piece to make sure you have the correct diameter for your system. Make sure to pick up three new slip nuts as well.
Wrap plumber's tape around all exposed pipe threads. Slide a slip nut over the unthreaded end of the new tail piece with the threaded part of the nut pointed out. Connect the tail piece to the sink drain and tighten the slip nut with the channel lock pliers.
Slide the remaining two slip nuts on each end of the new sink trap. Connect the sink trap to the main drain line coming from the wall using the slip nut to secure the sink trap. Attach the other end of the sink trap to the tail piece. Secure the sink trap to the tail piece by tightening down the slip nut.
Tighten down the sink drain flange using a pipe wrench to turn the retaining nut under the sink ¼ turn.
Tighten all of the connections between pipes under the sink using channel lock pliers.
Disconnect sections that will not stop leaking when tightened. Clean old plumber's tape off of threads and apply new plumber's tape. Reconnect drain lines using channel lock pliers.