When the driver pushes on brake pedal. lever action pushes a rod in to the brake booster and master cylinder. This produces hydraulic pressure in master cylinder. Fluid flows through the brake lives to wheel cylinders. The wheel cylinder uses this pressure to cause friction for braking.
As emergency or parking braking brake system uses cables or rock to mechanically apply the rear brakes. This provides a system for holding the wheel on hills or during complete hydraulic brake system failure.