When you depress the service brake pedal, two forces push back against your foot. One comes from a _____ on the brake pedal itself, and the second force comes from the _____ going to the brakes.

• A. damper / Hydraulic Fluid
• B. lever / Vacuum
• C. pad / Friction
• D. spring / Air Pressure

Answer: (D)

When you press the brake, you feel two opposing forces. One is from a spring that sits on or around the brake pedal to return it to its rest position. The other is the air pressure traveling through the brake system to the brake chambers. In heavy vehicles with air brakes, pressing the pedal opens a valve that builds compressed air pressure in the brake lines and chambers, and that pressure resists your foot as it moves the pedal, giving you the pedal feedback you feel. The combination of the pedal-return spring and the air pressure going to the brakes explains the two forces acting against your foot.