Brake pistons and stuck retaining pins

[Fazerider]

Possibly just crud on the piston. You can clean them by pushing them out a bit, one at a time, by using tyraps/ thin G-clamps/etc. to hold the ones you’re not working on in the cylinders out of the way. You’ll need to rotate the piston to get all the muck off.
Possibly, the seals need changing… particularly if you find the brakes drag excessively. Either way, if you’re doing it yourself, a brake piston tool makes it a much easier job.