Leaking pipes under the kitchen sink will leave water stains or even puddles on the floor of the cabinet. In some cases the problem can be fixed by simply tightening the joints on the sink plumbing, but often the only cure is pipe replacement. Modern kitchen sinks use PVC pipes which are fitted without soldering and can be joined to older metal plumbing where necessary.
Plug the kitchen sink and fill it halfway with lukewarm water. Open the cabinet under the sink, pull the drain plug and shine a flashlight on the sink plumbing as the water drains. Look closely at the joints and the U-shaped trap pipe for water leaks. Look for small beads of water if the leak is not obvious.
Loosen the joints on either side of a leaking pipe or joint. Use an adjustable wrench to loosen the coupling nuts on metal pipe joints or PVC joints that are too tight to loosen by hand. Loosen the coupling nuts on the ends of both pipes joined with a cemented sleeve joint. Take all broken pipes to a hardware store or plumbing supply center for replacements of equal size.
Perform a PVC trap pipe replacement. Place the new rubber gaskets flat against the inside bottoms of the coupling nuts. Fit the threads of the trap pipe into the coupling nuts from the adjoining pipes on either side and tighten the nuts by hand.
Join pipes together that don't have coupling nuts. In a well-ventilated room, spread a coat of PVC primer on the wider inside sleeve fitting. Spread a thin coat of primer on the end of the regular size pipe that will fit into the sleeve. Allow two minutes for the primer to dry. Spread clear PVC cement on the same two sections. Slip the smaller pipe end into the sleeve and turn slightly as you force the two pipes together. Hold the joint firmly together and wipe the excess cement from the outer ring. Allow 10 minutes drying time before using the pipes, or adding another joint to either of them.
Install a tailpiece and drainage arm directly under the kitchen sink. Place the inner ring of the tailpiece washer into the upper end of the tailpiece pipe. Push the tailpiece up against the bottom of the strainer basket and thread the slip-joint nut from the pipe around the threads of the strainer basket. Tighten the slip nut firmly by hand. Join the bottom of the tailpiece to the drainage arm near the elbow bend with a coupling nut or by cementing the joint. Tighten the straight end of the drainage arm to the coupling nut at the trap pipe joint.
Install a vertical drain pipe by connecting the bottom to a PVC drain with a coupling nut. Use a flexible coupling to attach the bottom to a metal drain pipe. Spread a thin layer of pipe thread compound around the top 1 1/2 inches of the metal pipe. Slip one end of the coupling over the top of the pipe and tighten the band clamp around it with a Phillips screwdriver. Push the bottom of the vertical drain pipe into the top of the coupling and tighten the upper clamp around it. Use a coupling nut or sleeve to connect the top of the vertical drain to the elbow or T-pipe connected to the trap.