Hi
I'm trying to figure out why my new toilet number 2 (briggs vacuum assist) toilet
does not do a full flush, (so far only once) all debreis has been cleaned out of the main line from the house and street side, this also was same issue with the last toilet american
which was installed summer 2003, house is 75 yo venting had been redone with 4" pvc -venting into the main line before the toilet. so I know theres no blockage with venting. now in this case the toilet drains into a 4" sanatary T (part of the horizontal main) (not a closet elbow)
and the line continues past the toilet transitioning to a 3" line at the T to the kitchen and tub drain. the tub does not have a vent of its own but it ties in with the kitchen drain that does have a 2nd vent.

This is all on 1 floor with in 15 feet the 2nd vent ties in with the main vent in the attic.

Im asking is it possable that as a syphon occures with the flushing of the toilet that air is being drawn from the 2nd stack, stiffling the syphoning action of the toilet?
or that its connected to a sanitary T than a closet Elbow
I have seen both toilet installations with using a sanitary T as well as a Elbow

Any thoughts
Thanks