Warnings here and there tell the seamstress or quilter not to blow into this or that. As careful as you'd try to be, your breath has moisture. Instead -- use canned air, or use a soft brush, or use a Q-tip. Keep a dry environment. No moisture.
Behold the photographic evidence.
The older bobbin case still works OK, doing whatever it needs to do with controlling bobbin thread tension, but I really don't want rusty parts. So I ordered a new bobbin case from my Brother dealer. I didn't tell him why. I've kept my guilty secret to myself -- until now.