Hi.
What I am about to say may not be all that helpful but perhaps give an insight into what may be a source of the problem. I purchased my Rune earlier this year. It was courier delivered to my home.
I found the back brake did not work at all !!
Having found there to be no brake fluid in the reservoir I removed the master cylinder off the from the bike and stripped it down. It was internally bone dry, not even damp, no evidence of brake fluid in there at all.
So at this point I'm thinking the back brake has never been operational. Hmmm.
It is a strange affair internally and to me this complexity will lend itself to blead back and possible air ingress. I didn't take a pisture of this assembly and now realise it would have been helpful. In appearance it is not unlike the the hydraulic flutings found in a hydraulic steering rack. Here is where I think the gremlin may be. Unlike a hydraulic sterering rack there is no preloaded fluid pressure created by a pump. A clue here is that a steering rack is designed to self purge and does so with because the hydraulic fluid is pressurised, I'm sure most of us have bled a steering rack so I won't get into this. It is this very aspect of the master cylinder design I believe may be the underlying problem; No fuid preload at rest. I have yet to reassess the brake and re-bleed the system to appraise the situation but the fluid level is, as filled by me. I believe the master cylinder is allowing a certain amount of leak back internally and since the seals are at their most effective when under pressure and since the system is pulled back by spring force a certain amount of negative pressure will theoretically be created internally. This theoretical internal vacuum may be enough to draw air past unstressed ( technically loose ) seals and into the master cylinder - just a thought and not impossible since a remedy written in this forum, having some pedal preload at rest supports the theory.
I will say though if my imaginative theory is correct then Honda need to issue a recall.
Regarding assembly, there appears to be an adjustable threaded rod linking the foot pedal to the master cylinder. Except it doesnt adjust, or it didnt appear to be designed to be adjusted with efforts to do so by myself leading me to conclude this. I have to say at this point I didnt have the workshop manual so did this job without reference.
In summary to the master cylinder all seals appeared in good nick but there was a small amount of dry and dusty residue of something inside the master cylinder. I cleaned everything, reassembled it and bled it. The system is the same as for a VTX ( I got one of these and have experience of doing this particular job ). It is, in my opinion very straight forward to bleed it. I got a firm brake pedal with sharp brake actuation and authority.
Roll on a month or so and it doesn't seem quite so firm, though I am bearing in mind I haven't ridden the bike on the road owing to needing a new front tyre yet.
I'm not particularly concerned about the brake pedal unless braking efficiency deteriorates then I will look for a permanent solution.
All the best
Howard