Tech advice - Brakes

I have an issue with my brakes that seems to have developed rather than been there from the start.

When I use the rear brake, the level never returns fully, causing an annoying rattle. This has worsened over time so I put it down to poor maintenance and maybe just the cables getting clogged up.

Yesterday, i lubes the cables with PTFE stuff, ensured they all moved freely through the top tube inner and made sure the handlebar cable was also free as the wind, which it all was.

Only slight issue was the amount of cable outer used at the rear, coming out fo the top tube and into the brake. This had always seemed a bit excessive and in refitting, I could feel more restriction than if this cable was just straightened out. I trimmed an inch off it and refitted with some improvement.

Now, the level returns all but the very last bit, literally a mm or two.

If I prise the rear brake open by hand, this last little bit releases as if something is preventing it from doing so, not much pressure required either.

Rear brake is a OEM Tekro-R720 in black and ive taken it off, cleaned and lubed it with no change. Ive adjusted it with the little screws used for alignment. These also have the effect fo tightening the spring but again, no real effect.

Are these just dodgy brakes or do people get the same issues with say, 105 brakes?