I had plumbers install 2 urinals in our company restroom. The are side by side, and one is lower for handicap access. When you flush the higher one, it sometimes fills up the lower one. I have had the plumbers back, 3 times, and replaced the urinals, but the problem keeps coming back. I am going to remove the drywall, and inspect the pipes, but I need to find a description, on how the pipes, and vent should look. Anyone?