I have a link under the bathroom sink that I'm having a hard time figuring out. The link is a pretty steady drip when the faucets are running. The leak originates behind the nut that screws onto the coupling from the pipe coming out of the wall. From the p trap a pvc pipe slides into a larger metal pipe that comes out of the wall. When I removed the metal nut I discovered that there was no washer, so I put in a plastic washer and it still leaked. I then put in a rubber washer and it also leaked. Kind of at my wits end. Seems like the washer should fix the problem. I checked and the pvc pipe is not cracked. Any ideas? Thanks in advance.