Brakes off
In the rest position (brakes off), the force applied by the return spring presses the diaphragm and plunger back to the rear of their travel. Because the diaphragm overflow valve is open, equal vacuum pressure will be present on both sides of the diaphragm, and the return spring in the master cylinder will be moved towards the rear of its travel. In this position, the central valves in the primary and secondary circuits are open and the brake system is pressureless. A stop pin prevents rearward movement of the secondary plunger, and a spring washer rearward movement of the primary plunger.
The master cylinder and other hydraulic components are filled with brake fluid under zero pressure.
The inlet and outlet valves in the valve block are in the rest position, i.e. the inlet valves are open and the outlet valves closed.