In a vehicle with S-cam brakes, what happens when the brake pedal is pressed?

Answer: B — The slack adjuster moves

When the brake pedal is pressed in a vehicle with S-cam brakes, the primary action is the movement of the slack adjuster. The purpose of the slack adjuster in an S-cam brake system is to convert the linear motion of the brake chamber's push rod into rotational motion that drives the S-cam. This, in turn, causes the brake shoes to expand and press against the brake drum, providing the necessary stopping power. The operation of S-cam brakes is essential for understanding how air brake systems work, particularly in heavy vehicles. The S-cam is designed in a way that, as the adjuster moves, it turns the cam, which then pushes the brake shoes outward. This is a critical aspect of brake engagement in air brake systems.