Your bathroom or kitchen sink may be susceptible to leaks over time that can wreak rotting havoc on metal and other drain parts nearby. In most cases, you should replace ruptured plastic or PVC pipes. It is also helpful to salvage plastic pipes by securely applying Teflon tape to pinhole leaks and stop them from reoccurring.
Shut off the water supply. Reach for the shut-off valve underneath the pipe needed to be repaired and turn the valve counterclockwise.
Disassemble the leaky PVC drain pipe. First, place a bucket underneath the pipe to catch any water spills. Then, loosen the nut connecting the drain line--or the elbow pipe--from its other connecting pipes from the bottom of the sink. You may need to use a pipe wrench to loosen the nut's force if you cannot dislocate by hand.
Loosen the connection of the pipe from the wall. Unscrew the plastic nut that connects the PVC pipe to the wall by hand. Be certain to hold the pipe assembly with your other hand to prevent its contents from falling and cracking.
Determine whether the new PVC and elbow pipe fits appropriately. Use a hacksaw if need be to cut the replacement PVC and elbow pipe down to size to ensure a snug fit to its connecting pipes. Re-attach the drain pipes--as suggested by Simple Home Repairs--in the opposite order from how you removed them. Connect the elbow pipe to the wall first, followed by the connection of the elbow pipe to the drain pipe (or the smaller L-shape pipe). Tighten the nuts securely around the pipe by fastening them clockwise by hand.
Test the drain pipe. Turn on the water valve and allow the sink water to run cold for a few seconds, with the stopper up. Check for any pipe leaks. Close the stopper and allow the sink to fill halfway up. Release the drain stopper and make sure there are no leaks. Leave a bucket underneath the drain for a few days to watch for water leaks. If leaks do appear, wrap a few layers of Teflon tape clockwise around the point of the leak to stop any future leaks.