Determining the cause of a plastic pipe water leak before attempting to fix the problem will save you from purchasing large amounts of replacement parts. Pipe damage and loose plumbing connections both cause cabinet-damaging water leaks under sinks. Both types of water leaks are identifiable and fixable without hiring a master plumber. Fixing a water leak fast ensures the cabinet holding the sink receives as little damage as possible.
Tighten the slip nut connections of all the plastic pipes by turning each slip nut clockwise by hand. Turn each slip nut 1/4 turn clockwise with a pair of slip-nut pliers after they are hand tightened. Wipe the surface of all plastic under sink pipes with a dry rag.
Place a plastic bucket under the sink pipes. Turn on the sink faucet. Allow the water to run while you inspect the plastic pipes for leaks. Inspect the pipes for leaks. Note any locations you find leaks and whether the leaks occur at or away from a pipe connection. Turn off the faucet and wipe the pipe with a dry rag.
Turn the slip nuts holding the leaking piece of pipe counterclockwise to loosen the connections. Push the loosened slip nuts away from the connections. Slide the tapered slip washers away from the connections. Remove the pipe from the under sink plumbing.
Inspect the tapered washers and slip nuts for damage. Replace the parts as needed. Ensure the thicker end of a slip washer faces toward the inside of a slip nut. Reassemble the under-sink plumbing with the new connection components unless the pipe is damaged.
Cut a piece of replacement pipe to the length of the damaged piece. Install the new pipe using new slip nuts and washers. Tighten the connection as earlier described. Turn on the faucet and inspect the repair for leaks. Repeat the process of inspecting and replacing pieces of plastic pipe, if you notice additional water leaks.