Re: Soil Stack
When you say "conventitial" I think slab, which mean you would not be able to crawl under the house and look at the piping. Now, you have said you have a crawl space so that is good for re-work. As far as the description of "seperate" tie-in, agian confusion.
Do you mean that the sewer pipe to the toliet goes out to the main sewer without any branches? And do you mean the tub and sink also tie in to the main sewer without any tie in to the rest of the house plumbing?
Also, being built in the 1930's is the pipe cast iron? How will you tie in the new PVC to cast iron? If the toilet is a slow flush right now, it may mean that it isn't properly vented. But, if it flush okay it would seem to me that it is vented. Now are you going to re-use the toilet from before. The reason I ask is because of the gallons per flush being 3.5 or higher is no longer needed and the newer standard is 1.6.
A couple things to think about about.
Process of elimination. Good luck.