I recently had a room addition done and part of the project was opening a pass-thru into the new room above the kitchen sink, which required moving the vent for the sink. The contractor put in a new 2" vent in the new laundry room approx. 8' from the sink vent and said it would serve both no problem. Well there is a problem in that now when the washer drains it draws the water out of the sink trap and the gases now come up through the sink drain. The drywall work is complete and painted......the contractor is no longer welcome on our property due to some serious structural issues uncovered by a new friend, in the construction trade. The structural issues have been resolved and I really do not want to have to tear out any more drywall. We are financially tapped out and cannot afford a plumber right now and do not want to ask for any more free help from my friend as he has a business to run and a family to support, so I need to be able to fix this myself. I have renovated homes for the past 20yrs, but always hired the plumbing and electrical work out, as I know my limits. Any suggestions on a fix for this problem. Thanks in advance, Michael