: The waste line, which is a 12 ft. run, does not allow enough slope to install a connection in the vertical position, so the Y is horizontal. I have also tried venting before the Y and this did'nt help. There were orignally different toilets with ordinary flush valves installed here and I was told they did not siphon like these ones with the flushmasters which seem to have a much stronger flush. What do you think about running a seperate waste line on one WC that would tie in farther down on the existing waste line.
: : If I understand your layout and you have the two WC's connected in a "Y" arrangement to a common waste line, putting a vent downstream of the Y connection won't help. You need a vent in each leg before they connect together.

: : : I have two toilets (with Sloan Flushmasters in them) that are next to each other and connect into a double elbow. When one is flushed it siphons the water out of the bowl in the other. I tried putting in a vent pipe right after the turn, it didn't help. Anyone have an idea?