I have worked in this area, the piston comes out fairly easily but to do so you have to relieve any pressure in the piston & push it inwards shoving out any residual oil, it closes easily then, so leave the pressure relief screw partially open. Remove the retaining clip etc obviously. I also found it useful to get a piece of wire & use it to keep the piston retracted when replacing it after putting on the new boot. You may find you lose a bit of hydraulic oil in the process, so get a bottle of it before starting, always useful for top ups in any case.
Regards
agd123
Regards
agd123