1. I have absolutely no idea of what your are trying to depict.
2. IF the system is acutally bell and spigot fittings as depicted, then it is almost IMPOSSIBLE to install the backwards.
3. I doubt that ANY pipe could "sag" 18" without breaking. That distance is more of an offset than a sag.
4. You appear to have a "bit of knowledge", but may not have enough to really be diagnosing this system, since little of what you indicate should happen in real life. Pictures would be more helpful than drawings, because they will tell us WHAT things look like, rather than what they SHOULD look like.