Friend purchased a 2 family deweling in hopes of making repairs and then resell the home. Me, who likes to think he can be a plumber is going to help out where I can. Noticed that in this old home there are zero vents, all the drains run into the 4" cast main drain. Should I keep it that way or run 2" vents for the two kitchen sinks and also do the same for the two bathroom tub/shower and sink? And why dry vent at all if this system works?