Plumbing problems in old house

[Dan Graney]

I just bought my first duplex and I am going to live in the upper unit to house hack. Need to get a few things fixed before I rent it out. Most serious is really slow running drains in the upper unit. If I run the water in the sink enough to brush my teeth it backs up into the bath tub.

I have tried draino but it didn't seem to make a difference.

Talked to a plumber and he wants like $500 to power auger the drains, including money to remove and replace the toilet.

So I am thinking of renting a power auger and doing it myself but I am concerned about the age of the drains. The house was built in 1920.

Found a good product that slowly eats the gunk in the drain lines over time that I will use to clean the lines further once they are flowing enough. Product is called Bio-Clean and it has many positive reviews.

Would appreciate any insight I could get. Thanks in advance.