A leaking sink trap is a major problem and needs to be addressed as soon as it is found. Water leaking under any sink, if not repaired promptly, can cause issues with mould, mildew, rotting wood, and possible cabinet and eventually floor replacement. There can be several causes for a leaking sink trap, but once you determine the problem, the repair is fairly simple. There are two types of sink trap materials: metal and PVC. The problems for each can be repaired the same way.
Place a bucket beneath the sink trap to catch any leaking water.
Plug the sink drain with a stopper, then fill the sink with water.
Pull the stopper from the sink drain and observe the water running through the sink trap. If there is a leak, you will see it in very short order.
Determine where the leak originates. If the leak is coming from a union, take your slip joint pliers and tighten down the couplings by turning them in a clockwise direction. If the rubber washers are good, the leaking will stop as they are tightened. If the leaking continues, go to the next step.
Remove the couplings by taking them off with your slip joint pliers. Turn them in a counterclockwise direction. Inspect the rubber washers and if they are bent, distorted, cracked or compromised at all, they need to be replaced with new ones. Once replaced, tighten the connections.
Leaking water that originates at the bottom of the sink trap means the actual trap itself is either cracked or rusted if it is of the metal type, but will need to be replaced regardless. Remove the couplings with your slip pliers and replace the actual trap itself by putting a new one on.