Slow draining waste line

[ibz1492]

I live part time in a mid-70's multi-story condo. When I return to the condo each year, the drain is particularly slow, even though units above me have had people in residence. A few months ago, the condo board asked a plumber to investigate. Plumbing company ran a snake down the 5-story stack. This blocked the drain 100%. A thick rust coloured mud oozed up from the shower drain (the lowest drain in my unit. I scooped up some of this material and discovered that it was composed of mostly ferrous material and dirt. This seemed to be the same material found inside the typical stack. Plumber claimed it was something different and insisted he had to rip up the shower in the middle of my unit. I said that I wanted to see the trap and the rest of the waste line that he removed. Plumber took the old waste line so no one was able to see it. Then last week a neighbor had a slow drain also. Same plumber came and camera'd the waste line and found it sagged. He then snaked the entire stack and thus loosened the fine debris in the stack which closed the drain entirely. He then talked the condo board into ripping up a 16 foot section of the waste that ran directly through the living room of the neighboring unit. That waste line (which I saw after it was exposed but before it was replaced) had an old, fine crack on the top surface of the pipe but no moisture at all around the pipe that was to be replaced. It turned out to be full of the same rust particles and dirt that the plumber had loosened when he snaked the stack. I am now super-suspicious; should I just go along with what the plumber says with no proof that he is being honest? I get the feeling that the plumber is milking an un-educated condo board. All thoughts and ideas would be most appreciated.

[cacher_chick]

There is no way for someone to diagnose this over the internet. Any drain line which does not run clear and empty is either plugged or has a section or sections which are not properly pitched. When a building drain is working properly, it will empty completely all the way to the municipal sewer.

If you don't trust the first plumber, you have little choice but to hire out for a second opinion.