As it turns out, the water fill lock was merely corroded from not having been opened in a long time. Some heavy soaking of WD-40, in addition to light taps with a hammer, released it. It is all lubed up and working fine.
I learned my lesson about not letting mechanical things go unused for long periods of time. As I get closer to getting my money pit (said with Love of course) rolling, I'm starting a routine of checking all functions. I don't want to hear, "Houston, we have a problem" anymore!
