Abstract: The present specification relates to a piston and cylinder arrangement for use in a vehicle disc brake. Such arrangements are either hydraulically or pneumatically operable. In disc brakes the piston and cylinder of the brake actuator are usually sealed against the ingress of dirt, by a flexible boot arranged between the piston and cylinder at the cylinder open end. Additionally a separate pressure seal is advantageously provided between the piston and cylinder. The present invention provides for a flexible boot which engages in retaining grooves provided on the outside of the piston and on the inside wall of the cylinder adjacent to the open end of the cylinder, the part of the flexible boot engaging in the retaining groove in the inside wall of the cylinder, being compressed radially of the cylinder to thus act as a pressure seal between the piston and cylinder. By virtue of this dual function, the piston can be shortened and the weight of the arrangement induced. Also assembly is simplified.