This is quality city water....not hard at all. It tastes as good as New York City water when I used to live there. This local city mails out annual independent lab analysis which gives very high marks. I only have a carbon filter for our ice maker, which reminds me I probably should replace that after being there at least 3-5 years. Have not had to replace any faucets/aerator filters or shower head nozzles in the last 10 years except
on a cosmetic high output "Rain" shower head. (Obviously 2.5 gpm restrictor was discarded)
I'm guessing this valve was 40+ years old based upon what it is going to in this 90 year old house, and knowing the occupancy history and date of improvements. No leaking sewage or ammonia or any other contributing causes. It was actually over a shallow cabinet of "nick nacks" that I had not looked at for at least 15 years, so the moisture mostly got absorbed into the top of the wood cabinet.
I have been getting only the 90 degree ball valve lever replacements put in....but I bet there are at least 30-40 remaining round valves all over. It's a crap shoot if you have to close one of these valves whether it will start leaking, and often tightening the packing nut will not help. I got into this whole mess when our 20+ yr old washer started leaking where the clothes drum is mounted on a rotating bushing. The first two sets of hot/cold water valves would not turn off the water supply completely, and 3 of them started dripping afterwards. I had to shut off the main coming into the house just to
replace leaking mixer connection valve for washer.