When I picked up Rosie from an abusive owner, the rear caliper and rotor (blue) were trashed. I installed a new rotor, caliper, line, and rebuilt the master cylinder thinking those problems were over.
After the shake down ride, I noticed the new rotor was hot, and I seldom use the rear brake. Hmmm. I found that there was still a little air in the system, and bleeding it completely cured that problem. I hung a 20 lb. weight from the brake lever for 24 hours, and that let the micro air bubbles percolate to the master cylinder resevoir when pressure is relieved. At least that's my theory..