Drainage in my bathroom sink has been slow to none for about 5 years. We replaced the sink and the pipes underneath, no help. The drain pipe ties into the main drain about 3 feet away and just inches above where the toilet joins. We have run a snake all the way into the main drain and found no clog. The vent has been checked and is clear. We had been using the sink with the plug in and bailing it out when it got full but anytime a sinkful was let out it initially appeared to drain then stopped. After plugging the sink again and bailing out the remaining water, pulling the plug again resulted in water raising back up. Recently we pulled the sink off again and the only evidence of water we found is in the P trap, further down was rust. What could cause the flow to stop at the trap?