When you take the caliper off disconnect the brake line. Take the pads out put a thin piece of wood in the place where the pads were install. then keep you finger out of the way and blow with compressed air in to the port where the brake line was connected. The piston will come shooting out and smash into the piece of wood. Don't have you fingers in the way. Inspect for rust, if there is no rust get yourself a rebuild kit and put it back together. If you pull you brake lever and the piston moves but will not move back after letting go, you might have some internal swelling of the soft brake lines. How old are your brake lines?

@@HeikosGarage Thank you so much for that piece of advice. I think you are also right with the sweilling of the brake lines as they are still the old ones from 79 and I anyhow wanted to replace them with flexible and new ones as well as put the brake fluid cylinder up on top of the handle bar instead of where it sits right now under the tank.

if the piston doesn't move then something during the rebuild went wrong. the piston should move freely. check Brooks write up about it. brook.reams.me/bmw-motorcycle-rebuilds/1975-r756-build-s-replica/34-bmw-r756-rebuild-master-cylinder-disk-caliper/