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.
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.