Retired plbg1
They top toilets have the biggest vent either 3″ or 4″. Sounds like you gave something in the stack are pipe that you open up with cable and when you take out cable it closes back up. Have a camera run through the pipe. Are all the tanks filling up to mark in tank, is there anything between tank and bowl. do they go down when you plunge the bowl, is there something in bowl, take one up and turn upside down and flush out with hose, try 5 gal. bucket of water see if it flushes. After you take up bowl have somebody flush another toilet and see if it backs up be ready to close the flapper.
Art retired plbg
