Last week we discovered our basement shower and toilet had some lovely sewage back up. We called our local rooter service who snaked and videoed the discharge sewer line to the street and said the line was clear, I assumned the snake cleard the block. We thought the problem was solved yet when we ran the washing machine in the basement yesterday, the washing machine discharge over flowed and some of the dishcharge wound up in the slop sink next to the washing machine. I'm thinking it could be the vent line? I can see what I think is part of one running vertical next to the sewer discharge line. Most of the vent line is behind a sheetrock wall, and I can't tell where it goes. How would a plumber check the vent line?