Tell me if the following makes sense. My neighborhood is rather old, but on city sewer. Some homes have basement baths, most don’t. When the rare sewer backup happens, the poor neighbors with the sewer tied basement baths, start to get EVERYTHING coming back from the city line (uh oh), lowest point and all that. Had one neighbor who got pissed, and switched TO the el cheapo (right behind the commode) unit, to avoid anything coming BACK from the city side during a main line backup. I’m guessing that would work right ?? Some kind of check valve where the hit the main in the house would prevent that ?