A force transmission device for a disc brake operable to move a friction element of the brake into engagement with a rotary brake disc along a first axis in response to a loading from an actuating member, the device comprising: a guide member having a generally cylindrical guide form; a piston comprising a complementary generally cylindrical outer sliding surface slideable within the guide member, wherein; an anti-rotation formation is provided between the guide member and the piston, the formation comprising a recess extending generally parallel to the first axis and a complementary projection.