A caliper comprising: a piston bore and a seal groove located in the piston bore proximate to an open end of the piston bore, the seal groove comprising: a front wall; a rear wall; a bottom wall spanning between and connecting the front wall to the rear wall; a projection in a central region of the bottom wall suitable for allowing twisting of a seal during a brake retract and/or brake release; wherein the rear wall diverges away from the front wall as the rear wall extends away from the bottom wall and towards the bore surface.