Michigan CDL - Air Brakes Practice Exam

What happens when the S-cam is turned after pressing the brake pedal?

1. The brake shoes retract

2. The S-cam stops working

3. The brake shoes press against the drum

4. The air pressure is released

The correct answer is: The brake shoes press against the drum

When the S-cam is turned after pressing the brake pedal, it plays a crucial role in the operation of air brakes on vehicles. The S-cam mechanism is designed to convert the linear motion of the brake chamber push rod into a rotational motion. As the S-cam rotates, it pushes the brake shoes outward against the brake drum. This pressing action creates the necessary friction to slow down or stop the vehicle. This process is essential for effective braking, as the friction created between the brake shoes and the drum is what allows the vehicle to decelerate. The design of the S-cam and its interaction with the brake shoes is a key factor in the air brake system's performance, ensuring that braking is both efficient and reliable. Other options, such as the brake shoes retracting or the air pressure being released, do not occur at this stage of the brake application process. Instead, the focus is on how the turning of the S-cam facilitates the contact of the brake shoes with the drum, which is fundamental for achieving the braking action required for stopping the vehicle safely.