Michigan CDL - Air Brakes Practice Exam

How does pressing the brake pedal affect the air pressure in S-cam brakes?

1. It reduces the air pressure in the tanks

2. It increases the air pressure in the tanks

3. It displaces air into the brake chambers

4. It maintains the current air pressure

The correct answer is: It displaces air into the brake chambers

Pressing the brake pedal in a vehicle equipped with S-cam brakes initiates a crucial process that results in the displacement of air into the brake chambers. This is how S-cam brakes operate: when the driver presses the brake pedal, it activates the air brake system, causing air to flow from the storage tanks into the brake chambers. Once air enters the brake chambers, it applies pressure to the diaphragm inside the chamber. This diaphragm pushes the pushrod out, which in turn rotates the S-cam. The rotation of the S-cam forces the brake shoes to press against the brake drum, effectively slowing down the vehicle or bringing it to a stop. Therefore, the displacement of air into the brake chambers is a key step in engaging the braking mechanism. In contrast, the other options do not accurately describe the fundamental operation of S-cam brakes. The process does not involve a reduction or increase of air pressure within the storage tanks either, nor does it maintain current air pressure; it specifically engages the movement of air necessary for braking.