There are many connections on the drain under a typical sink and each one has the potential to leak. The sink drain pipes are all tightened by hand so that they can easily be taken apart and so they can slip inside each other for easier assembly. Sometimes they may be tightened with channel locks which will make them harder to loosen. Bathroom sink drain pipe leaks are often caused in these connections when they become loose or bent from being hit by things put under the sink. If one of the connections is leaking you may be able to fix it by simply loosening the fitting, pushing the two pieces together and then tightening it. If that does not work you may have to take the fitting apart and clean the fittings and gaskets and then put it back together. If one of the pieces is cracked and leaking, replace it with new parts found at any home hardware store.
Before taking the plumbing apart you should try to isolate the leak to a specific area. Remember that water runs down hill and the leak may be in a different spot from where the drip is.
how the drain system plumbing works
The drain system for a bathroom sink consists of:
The drain system connections consist of compression fittings and regular screw on fittings. A compression fitting has one end that slides into the other instead of screwing together. It makes a seal by compressing a ring onto the pipe, it is threaded, but the threads and nut are on one pipe and together they squeeze the other pipe. The pipe with the threaded nut squeezes the smaller pipe and creates a water tight seal. Leaks with compression fittings are caused when the nut becomes loose or cracks.
drain flange: The drain flange connects the drain to the sink. It is inserted from the top of the sink and a nut is threaded on from below. Leaks at the drain flange happen when the nut holding it on becomes loose, the flange becomes corroded or sometimes when the plumbers putty deteriorates. If you take the drain tube apart, make sure you use plumbers putty where the sink touches the drain flange, roll out a snake roll about a quarter inch round and put it around the flange and then tighten the nut so it compress and squeezes out. Also make sure the hole for the horizontal stopper rod is pointing to the back of the sink.
drain pipe and stopper lever: The drain pipe connects to the bottom of the drain flange. It holds the lever for the drain stopper. Both of these connections are screw on and have washers in them just like a garden hose. They often leak when the nut becomes loose or the washer is damaged or worn out. If they are leaking first try to tighten the nut. If tightening does not work then replace the washers.
p-trap: All drains in a home have a P-trap. A P-trap is a "U" shaped section of pipe that holds water in it so sewer gasses cannot come back up the drain into your home. It also may catch small items like a ring if they are dropped down the drain. Sink drains have a P-trap that is within a foot or so of the drain. Check the nuts on the P-trap for leaks. If the connections on the P-trap are leaking try to tighten the nuts by hand. If that does not work then remove the P-trap and clean the the fittings and reinstall it. If that does not work then it is time for a new P-trap.
horizontal pipe to the wall: The horizontal pipe to the wall goes from the P-trap to the wall drain. Make sure that the horizontal drain is sloping toward the wall away from the P-trap even if very little. Also, make sure that the compression fittings at each end are tight so they don't leak. You can increase the slope on the horizontal pipe by raising the P-trap up. If sloping and tightening the fittings on the horizontal drain pipe do not stop the leak then it should be replaced along with its fittings. Sometimes the compression fitting that seals the connection between the p-trap parts wears out and will no longer seal.
These are the most common areas of concern relating to leaks around the bathroom sink drain pipes and stopper systems in sinks.