What component does the rod move after the brake chamber fills with air?

The correct choice is the slack adjuster because it plays a critical role in the functioning of the air brake system. When the brake chamber fills with air, the increased pressure pushes the diaphragm within the brake chamber. This action moves the rod, which in turn rotates the slack adjuster.

The slack adjuster's primary function is to ensure that the brake shoes are positioned correctly against the brake drum. As the slack adjuster moves, it adjusts the distance between the brake shoes and the drum to compensate for wear, ensuring that the brakes engage effectively. This adjustment is essential for maintaining optimal brake performance and safety in vehicles equipped with air brakes.

In contrast, while components like brake pads, brake calipers, and piston assemblies are indeed part of the broader braking system, they do not directly respond to the movement of the rod after the brake chamber fills with air in the same way that the slack adjuster does. Understanding this mechanism is crucial for ensuring that air brake systems function correctly and safely.