Galvanized pipe will often "fill" with corrosion and debris over time, gradually closing off the pipe and lowering water flow kind of like a clogged artery.

What a relief I felt reading your reply:)Quick clarification: For the Schedule K copper, likely 1" line, silver soldered where underground, for water main and irrigation, with brass valves, wrapped in black foam jacket before they backfill- Does the copper need a plastic sleeve too to prevent potential corrosion from the soil (or from grass fertilizers/pesticides/stuff in lawn/garden), or is that what the black foam jacket is for? 

There's corrosion present, an inoperable thermostat, and it's about 20 years old at this point.

I also use dielectric grease on the battery connections to prevent corrosion.

Soot left on metal surfaces can cause pitting and corrosion, example faucet sets, door knobs.  

Aloha,Leaks in the wall or subfloor are always going to be Owner responsibility, as the cause is most likely age and corrosion (which often causes cracking of the iron due to expansion) of the pipes themselves or failure of no-hub bands typically found at prior repair points.

Just for the 2 units that in my mind were rent ready (have needed plumbing work in both). 1 water heater wasn't working (not producing any hot water, eneded up replacing 2 thermostats and a lower element). the other one required a new dub drainage connection (it was completely rusted / corrosive damage and was leaking into the basement. luckily this was on the first floor.

We had main riser leaks due to old copper pipes that had corrosion and were springing pinhole leaks likley due to disimalr metals being used in the system creating "galvanic corrosion". it was a big job but we did it all ourselves. went ahead and replaced the main hot and cold risers from the boiler room to the third floor.

The building is being sold as-is, there is no immediate issues that alerted me as the owner that we even had issues and I’m told that pipes with corrosion are common and can still flow because the dirt below becomes compacted and does not impede the flow.

It has a lots of corrosion to the point that the camera can't tell where the leak coming from.