I’ve lived in an eighty-year old house for four years now and still have the same problem. When I drain the kithen sink, sewer gas and sometimes the kitcen sink water comes up. Only after being snaked, the problem goes away temporarily but returns promptly. I don’t let ANYTHNG go down the sink except soap and water. The plumber says there’s nothing blocking the vent and now the shower drain is slowing up too. I hate to call the plumber again because he doesn’t believe me anymore. Could it be that the snake is only clearing a small passage through the blockage and not completely cleaning the pipes? I’m on city sewer.