Which of the following components would be a part of a hydraulic brake system?

The master cylinder is a fundamental component of a hydraulic brake system. Its primary function is to convert the force exerted by the driver on the brake pedal into hydraulic pressure, which is then conveyed through brake lines to the brake calipers or wheel cylinders at each wheel. When the brake pedal is pressed, the master cylinder creates pressure in the brake fluid, allowing for effective braking across all wheels. This hydraulic action is crucial for the operation of modern braking systems, providing high efficiency and responsiveness.

In contrast, the S-cam is typically found in air brake systems and is involved in the actuation of the brakes through a mechanical linkage, while the kingpin is a pivot point for the steering system of a vehicle. The pitman arm also relates to the steering system, connecting the steering gear to the drag link. Therefore, the master cylinder stands out as the correct answer, as it is specifically integral to hydraulic brake systems, while the other components pertain to different mechanical functions.