On my coach there are six drain points for moisture in the air system 4 tanks and two filter/collectors. When I purchased the coach I rebuilt the air dryer which had not been apart in 120K miles.
The two filter/collectors were the same way. When I ordered replacement filters the box they came in looked like it came over on the Mayflower. I wonder how many owners are aware that there are filters in the system and how many actually get changed. My auxiliary air filter was almost plugged with debris and rust.

I rebuilt a lot of my norgren valves in place without even removing the air lines. I believe that most of them fail due to lack of lubrication, once the assembly lube goes away they run dry rubber on a dry bore and therefore wear away the sealing surface.