Ok, iv"e asked a few questions here and still can't figure my problem out. So here is the whole story. Replaced old cast sewer line in house, cleaned the roof vent line to make sure it is open. When you let water out out upstairs tub, the toilet in that room and on the floor below blow bubbles and the one below backs up.After a bit(10 15 min) the toilet drains back out. We replaced the line because it was decyed and open in areas, we never had flushing or draining problems before we replaced the line. I wonder if the vent line is too small diameter for the new sewer line.. Thanks for any help