In the most common type of foundation brake, the s-cam drum brake, what actually forces the brake shoes outward into the drum's inner surface to stop the wheel?

Explanation

In an s-cam drum brake, pressing the brake pedal sends air into the brake chamber, and that air pressure pushes a rod that moves the slack adjuster, turning the brake camshaft; the s-shaped cam then spreads the brake shoes outward until they press into the drum's inner surface, creating the friction that slows the vehicle. This mechanism is standard driver-training content describing how the required air brake nomenclature under 49 CFR 383.111(b)(1) actually works; the paragraph names general air brake system nomenclature as a knowledge area but does not itself spell out the s-cam's specific mechanism. A wedge driven directly between the shoes describes wedge brakes, a less common foundation-brake design, not the s-cam type. Spring pressure alone describes what happens when spring brakes engage during parking or a loss of air pressure, a different system from the service brake's normal stopping action. A hydraulic piston is wrong because air brakes use compressed air, not hydraulic fluid, to apply force, which is the basic distinction between an air brake system and a hydraulic brake system.

Source: 49 CFR 383.111(b)(1)