It can be a serious mess. I gather that 1/4 per inch slope is recommended as greater slope will supposedly allow the water to outrun the solids and leave them behind. And then there is the new issue of not enough water to take the stuff away.

But if one is stuck with bad lines, wrong slope, bottlenecks, whoa, replacing those is a serious challenge. One of my best clients lives in a house at least 80 years old. The plumbing is old and weird. I've replace problem bits of it with ABS but plenty is still old and funky. It's a crawl space so some of it could be replaced but dang, doing that hard work in a crawl space is a chore.

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1986 300SDL, 362K
1984 300D, 138K