I have a washing machine in my basement. The existing drain line is higher than the machine. The existing drain line is 2" ABS pipe mounted on the wall 5 feet above the floor and behind the washer. The drain line is a horizontal 2" line that connects to a 2" P-trap with 12" long vertcal riser. The flexible drain hose from the machine is hooked over the top the riser. 5 feet from the P-trap the horizontal 2" pipe connects to a vertical vent pipe that drops immediately (6"drop) into the 4" building sewer. The building sewer exists through the foundation wall and connects to the side sewer. The riser top overflowed recently, probably due to some lint build up, and this caused me to review the situation. It seems to me that the short riser is a weak point in my drain. It really can't be lengthened because the pump is already pumping up to a height of 6 feet. I was thinking of making a closed connection. What I was thinking of doing was sawing a few inches off the riser and fitting a wye piece. I would connect the flex hose to the branch using a nipple and clamp, and whatever reducers. To the top of the wye I was going to connect an air admitance valve. The valve is to break the suction if it ever gets into a syhpon back into the washer mode. Does this sound like it will work? Also I am a little worried about forcing the water out of the P-trap - do you think that could happen?