I live in a tri-level home with one bathroom in upper level and one in the lower level, each with pressure assist toilets. The bathroom in the lower level, when flushed, doesn't fully empty the bowl. When the toilet in the upper level is flushed, it pushes air out of the lower toilet and into the bowel, splattering water from the bowl onto the seat and bottom of seat cover. I pulled the toilet in the lower level and could not find anything stuck in the toilet bowl. The toilet waste drainline is empty. When I run water into the drain, it does not fill up. When I flush the toilet upstairs, I can feel it push just a small amount of air out of the floor drain. What could the problem be?

Sincerely,
themorgan5

"When the toilet in the upper level is flushed, it pushes air out of the lower toilet and into the bowel, splattering water from the bowl onto the seat and bottom of seat cover."

This tells me that you have a partial blockage downstream from the downstairs toilet. Let me explain. When you flush the discharge rushes down from upstairs and hits the blockage and bounces back sending a bubble of sewer gas ahead of it. This is what comes up in the lower toilet.
The solution is to snake the drain downstream from the first floor toilet. This can be done in several ways. (1) you can pull the toilet and snake from the open closet bend**or** (2) You can snake from the downstairs lavatory roof vent. Put out enough cable to reach the base and 25 foot more. Test by flushing the upstairs toilet. Good luck, Tom