re: Anybody familiar with the early Kawasaki Brute Force?

Not always.

They can collapse internally if they are metal outside. Like where they turn into the wheel.

They don't have to completely collapse. Just enough for restriction.

Bleed all your brake lines and leave the master cylinder open but covered to keep dirt out.

Try doing the exact same thing you did and see if it locks up.

At worst you flushed old shite out and will have to just refill and bleed.

Don't spill brake fluid on anything painted. It's corrosive.