Michigan CDL - Air Brakes Practice Exam

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

1. The brake shoes retract

2. The slack adjuster moves

3. The parking brake engages

4. The brake lights activate

The correct answer is: 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.