In case of a hydraulic failure in the brake system served by the primary piston, the primary piston will move forward when the brakes are applied but will not build up hydraulic pressure. In this case very little force is transferred to the secondary piston through the primary piston spring until the piston extension screw comes in contact with the secondary piston. Then, pushrod force is transmitted directly to the secondary piston and enough pressure is built up to operate its brakes.