What is the most common type of foundation brakes found on heavy vehicles?

• A. S-Cam brakes
• B. Air disc brakes
• C. Hydraulic drum brakes
• D. Hydraulic caliper brakes

Answer: (A)

Foundation brakes on heavy vehicles are built to work with the vehicle’s air brake system, and the most common type is the S-Cam drum brake. The S-Cam mechanism uses air pressure to turn a cam into an S shape, which forces the brake shoes outward to press against the inside of a brake drum. This drum-and-shoe design is rugged, easy to service, and well-suited to handling heavy loads across many axles, making it the standard choice in fleets for decades.

While there are other brake types in use—air disc brakes in some newer or premium setups, and hydraulic drum or caliper brakes on lighter equipment—the traditional and most widespread foundation brake on heavy trucks remains the S-Cam drum brake because of its reliability and compatibility with the air-brake system.