Hi Tom, here's an update. Sewer line from house to tank augered out, Ran hose in line for 15 min while cleanining it out. Filled tub over half full.. Upon draining everything was fine. Tub used again two hours later drains fine, I Use two hours after that, Downstairs toilet does not back up, but big bubbles come int o that toilet.. Coud it be that the vent is not good (big) enough for the tub? This is driving me crazy! Thanks for any and all help.