I also wanted to add that I moved into this house a couple years ago and it was built in the 50's so according to all of the diagrams I see, I don't think everything is vented properly. On all of the new house, I always see many vents, my house doesn't have this. That was why I desided to replace the bathroom sink and add the went in the first place.
This is what I think my house has in terms of plumbing drains/vents
There were only two vents that went out through the roof
-4 inch cast iron
-2 inch cast iron
plus my new 2 inch pvc (that I am not sure is working properly)
The main 2" vent is also the main drain for the:
-kitchen sink (drains to 2" stack and also has a vent that ties into the two inch stack higher than everything
-upstairs bathroom bathtub (is less than 1 foot from stack) basically has a trap and then goes into the 2"drain/vent stack lower than kitchen vent tie in and higher than kitchen sink
downstairs it is hard to tell because although there is a basement in the house, the part where the bathroom and laundry room are is on a concrete slab.
I think that everything down here including the bathroom sink, bathtub, toilet tie into the main 4" cast iron drain. Also the laundry room washer and sink tie into the main 4" drain. I can tell that there are no vents on the laundry room sink and washer because I can see them come up from floor.
The bathroom sink from upstairs drains into the same 4" that the toilet does and is within two feet. with a 2" to 4" Y.
Unfortunately, the only access I have to the downstairs level is a medicine cabinet. I could maybe also do something with the washer and sink if I were to open up my garage wall but I don't have access to the bathtub downstairs or even the toilet. I should draw a diagram but there are a number of unknowns. I don't know what they did back in the 50s but I am certain that this house is not plumbed correctly at all. I am just wondering how bad it is and if I can fix my bathroom sink problem and I also have the option of changin the upstairs bathroom sink and doing something different with the plumbing.
Apologize for the rambling but this is very hard to describe because I don't know the terminology or what is right/wrong. I will try to draw out a diagram tomorrow.