I have a toilet drain on the 2nd floor directly above a 2x6 wall on the 1st floor. Can I run the 3" drain from the toilet flange straight down to the basement without any elbows at the top and tie into the horizontal drain line? I have a 2" vent tied into the 3" pipe about 18" below the toilet flange and vented up throught the roof. Someone said that they think this design might suck the water out of the toilet trap. Is this true?