I just replaced the p trap on my bathroom sink with all new pvc pipes. The only leak I am getting is at the ptrap where it connects to the down pipe from the wall. It leaks a tiny bit after the water has run. No leaks when the faucet runs just after it sits a bit. When I removed the ptrap to try and retighten it was full of water. Is that supposed to happen?

The Ptrap is absolutely supposed to be filled with water. That is what keeps the sewer gasses from the main drain and the city/town sewer from coming into your home...;)

Use some pipe dope or Teflon tape on the threads and see if that helps with the leaker...;)

Mark

The trap is normally fitted with a ball joint type fitting that sometimes requires an extra tighten. On the down side, there should be a small compression washer. Make sure that is in place and that the joints are tight and use some Teflon pipe dope too, as Mark suggested..

As I understand it the issue is between the "J" bend and the weir of the trap that goes into the wall. This is a ground joint connection. Do not put pipe dope on plastic threads. The threads it makes slip and not hold. My guess is that the leaks because you have it crossthreaded Start the threads again. Good luck, Tom

If this is a PVC Ptrap with a union fitting (see image), a light coating of pipe dope or rectorseal thread sealant is appropriate for the trap threads and the face of the ground joint...Just FYI!