Old plumbing seems still connected?

[JeremyBartlett]

I have a customer with a flooding basement problem. This is a very old house in an older Seattle neighborhood, but all the plumbing has been updated in the last few years, all the way out to tbe city connection. There is a drain in the basement floor, as heavy rainfall will definitely find water entering the basement. I assumed it was simply a floor drain for this purpose.

However, flushing the toilet causes the water in this drain to surge, as does rapidly emptying a kitchen sink. When the washing machine drains, it floods the floor.

Why is this part of the original drain system effected by the toilet, when its clear all lines were replaced?

 

[Mr_David]

Clear to who? The basement drain is old system? From your description, It's pretty clear they tied the basement to the new and you may have a partial stoppage in the main.