Our kitchen sink won't drain properly. It is a double sink, the two sinks drain in to one pipe which then goes to the P trap. Then that goes to about a 3 foot pipe that kind of runs horizontal then it hits a tee and goes down. There is a vent in the long pipe after the P trap. Now, when you dump alot of water down one sink real fast--like pulling the plug--it will start coming up in the other sink. Then SLOWLY start draining down. If you have the plug in one side and run water in the other side it will barely drain, and it blows bubbles up the side with the plug in it. We have taken the pipes off at the sinks and they are clean. Now if I take the vent out it drains fast, but then pukes water out the hole where the vent was. It seems if I loosen the vent a little it helps some, but still not good. Liquid cleaners don't do anything. Any ideas????? Thanks.