In U.S. patent application Ser. No. 582,052, and now U.S. Pat. No. 3,967,536 incorporated herein by reference, a power braking system is disclosed whereby a pneumatically operated pressure differential servomotor is connected in series to a hydraulic servomotor to supply a master cylinder with an operational input force in response to an operator input. A cylindrical body in the hydraulic servomotor is pressed into a retainer in the hub of the movable wall of the pneumatically operated servomotor. During normal operation, the output of the pneumatic servomotor moves an output piston in the cylindrical body of hydraulic servomotor and when an additional output force is needed, a hydraulic valve is actuated to add the output of the hydraulic servomotor. Under some conditions, such as cold weather where the viscosity of the hydraulic fluid has been reduced or independent actuation of the hydraulic servomotor has occurred, a separation can occur between the cylindrical body and the hub. After such a separation, it is possible to lose hydraulic fluid through the actuation port of the hydraulic valve. It was suggested that the cylindrical body be fixed to the hub through a weld or screw thread arrangement. However, some relative movement is desired since the reactionary force from the master cylinder is carried through a disc without being modified by the hub.