A piston-cylinder unit for use in a disc brake assembly includes a cylinder and a hydraulically actuated piston received in the cylinder for axial movement therein. The unit has a flexible seal member with its opposite ends attached in sealing engagement to respective axially opposed portions of the piston and cylinder for elastic expansion and contraction in the axial direction. The flexible seal member and the opposing portions of the piston and cylinder form a chamber for receiving fluid pressure for actuation of the piston.