What device multiplies the brake force and decreases the effort required from the driver?

The device that multiplies the brake force and decreases the effort required from the driver is the power booster. This component uses either hydraulic pressure or vacuum assistance to amplify the driver's input when pressing the brake pedal. By doing so, it reduces the amount of physical effort needed from the driver to achieve effective braking.

In modern vehicles, power boosters are essential for creating a more responsive braking system, allowing even lighter pressure on the pedal to produce significant stopping power due to the booster’s ability to enhance the force applied. This is particularly important in situations where quick and strong braking is necessary for safety.

While other devices such as the vacuum assist booster also play a role in this process by enhancing the force applied to the brake system, the term 'power booster' is commonly used to encompass those types of assistive devices for braking. On the other hand, the output rod is typically part of the linkage system within the brake setup, and the return spring primarily serves to bring the brake components back to their neutral position after braking, rather than multiply input force.