A hydraulic servo for a brake in an automatic transmission has a piston secured against rotation without provision of a protrusion on the piston engaging the transmission casing. The piston of the hydraulic servo has an inner cylindrical surface configured to grasp, along their lengths, return springs compressed between the piston and a return plate to which the return springs are fixed. The return plate is bolted to a bulkhead portion of the transmission case and, thus, the piston member is fixed to the bulkhead portion and thereby prevented from rotating.