My daughter's house has an old 3" cast iron main drain/vent pipe. Her drains are running really slow. I have access to the bottom of the pipe and have pushed up a piece of pipe to see if there is a clog. There is a lot of build up in this pipe. Any way to clean this out? I am thinking of cutting down an chimney brush and running it up and down, would make a real mess though. Any other ideas?