I have reasons to believe the problem lies elsewhere.

In my house I have this pipe that's connected to the system. This pipe is capped with some sort of de-pressuriser. This sometimes smells like sewage. Could this be the culprit?

I am unsure of this, I am outside of the US. It's clear when there is a blockage that the flow of the flush is worse.

Yes, it's old and it's very probable that it's cast iron.

Mind if I ask a plumbing question?

Under my basement I have a bend that frequently gets plugged.

I have to flush a lot of water from inside and also from outside (I have a big pipe under my porch).

Is there anyway to prevent this?

I notice the blockage when I flush my toilet and the drain in my washingroom starts to bubble.