This Thread Should Move to Mechanics sub-forum...
...nevertheless, I'll persist.
BR-5800s are excellent calipers. Great. So, please provide a photo that shows the cable run from where it exits the rear stop or rear exit of the top tube to where it meets the caliper stop. Take one photo with the brake lever at rest, and then another with the brake lever fully squeezed. What changes?
Please also show a close-up of the mounting bolt from behind the rear bridge.
This shouldn't be as hard as you're experiencing...