I have a powder room and adjacent bathroom toilets that siphon from each other when flushed. Both toilet waste pipes are connected with a "T" to a 3" vertical vent pipe below the post-tensioned 8" concrete slab which continues down to the sewer pipe. The vertical vent pipe is reduced to 2" above the floor and continues up to the roof with four 90 degree bends. There are 8 other homes in my community built by the same builder with the same identical problem. The builder refuses to correct the problem in our homes because they and the City Building Manager say the plumbing codes were followed. This appears to be true except I haven't been able to confirm the # of 90 degree bends.

Any recommendations on how to correct the siphonage problem?