Master cylinder , few years ago a mate with a r850r had similar issues, new pads ,pins etc didn't cure it , even put on a new braided line in case the old rubber one was breaking down and swapped the caliper from the rear of my 1150gs and it was still binding
Bit the bullet and pulled the rear master cylinder apart and it had a ridge inside caused by a partial seizure (lack of use and water in the brake fluid) , fitted and new rear m/cylinder with the original caliper and it was fine afterwards
Bit the bullet and pulled the rear master cylinder apart and it had a ridge inside caused by a partial seizure (lack of use and water in the brake fluid) , fitted and new rear m/cylinder with the original caliper and it was fine afterwards