I know nothing about line locks, but the "regular" braking system is easy to trouble shoot. But I almost never bother on old cars. I simply go ahead and replace everything with NEW (not rebuilt) items and all is fine for more decades to come.

The dark fluid sure looks like you've been contaminated with water (moisture), and you probably have rust in your lines, and the PV may be clogged, or simply off center. No matter what, you need to get it cleaned out. A few bucks spent on new rubber and steel lines is a good start, then you can find the other problems as you go (frozen calipers, bad PV, etc.).