So I conclude from your note that the problem is either wear or corrosion in the cable-operated caliper mechanism (don't see how changing the brake fluid can have fixed things). I have often thought it was the cable, but have had to change one caliper due to the crappy mechanism, I remember at the time wondering if there was an aftermarket caliper which was better designed. Thought I saw something on the various brake sites, but after it went through the MOT (just) I had other things to worry about.

In seeking to undo one riddle, and find the true, I knit a hundred others new.