WPC, I agree if the joints and this system is NOT PROPERLY Vented a problem could occur.
Any time you work on older piping there is always the possibility of something failing.
This is the major reason I refuse to send out a helper/apprentice to do a mans job LOL. Besides its illegal in most of the better localities to send out a non licensed gentleman even in drain cleaning some states demand a license.
Why take a chance with an unskilled/unlicensed person who honestly may not have the skills to know what is the proper way to proceed?
Even drain cleaning can have its moments.