unsucessfull bleed of rear brake m821 2017

I have virtually non-existant rear brack; very long travel on peddle.
Guy checking for MOT let me off. I really don't use much rear brake; so never really noticed.

Have tried to bleed several times, but no joy; just does not firm up.
Tried going for run; killing via kill switch; then bleeding but nothing. First bike i have had with ABS; but I am sure I am bleeding right.

Under "normal" operation if I pump brake peddle; i can here a hiss from the side of the bike; approx under side cover where there is a fill coolant cap? This sound like a leak somewhere? What do you think?

Anybody any idea how I can fix?

Read lots of posts about heat damaged fluid; unmounting caliper to bleed; changing master cylinder; but....

Regards,
Owain