In an air braking system, which device directly regulates the pressure used to apply the brakes?

• A. The air compressor
• B. The brake pedal
• C. The parking brake
• D. The reservoir

Answer: (B)

Pressing the brake pedal is what directly controls how much air pressure is sent to the brake chambers. When you push the pedal, it operates the service brake valve (the brake control valve), which meters and releases air from the reservoirs into the brake lines. The more you press, the more air pressure reaches the brake actuators, increasing braking force; releasing the pedal reduces pressure and the brakes release. The air compressor supplies system air, the reservoirs store it, and the parking brake uses air (often with a spring release mechanism) to hold or apply the brakes, but they don’t directly regulate the pressure used for normal braking.