I am dealing with a 10 story building that is fairly new. They have been having laundry suds backing up from day one into a couple units at the bottom. I have already repiped some lines to create a "suds zone" and still having problems. The last time I was there I opened a cleanout and noticed a very strong wind traveling upstream through the pipe that I assume is coming from city sewer, the cleanout is near the exit of the building. Of course I can't see relief vents due to walls and ceilings and nobody wants their new condo ripped apart. There is relief vents shown on the blueprints. I was wondering why there is such a strong positive pressure coming upstream, constantly, and is that causing the suds to back up. This pressure is there even with very little water going down the drains. I don't think a P.A.P.A. is the answer as this pressure is always present and not from "positive transients". Any thoughts would be welcome. Already snaked and camera the lines, everything is clear.