Recently installed anew bathroom in an old farm house.couple of months after use ,water started backing up the vent pipe also new.I can loosen the clean out plug and everything works fine.What might cause the vent to pull water out of water closet. Keep in mind it is very small pluming is close.
Boy thats a tough one. The piping HAS to be installed WRONG. the vent obviously doesnt vent and your clean out plug becomes your vent when you loosen it. is the vent really wide open to atmosphere? no plugs or mis-placed traps? the vent sounds dead plugged.