Vehicles with air brakes must have:

• A. A tachometer
• B. An air pressure gauge to show the pressure available for braking
• C. A fuel gauge
• D. An odometer

Answer: (B)

Air brake systems depend on stored compressed air to apply the brakes, so you must be able to see how much pressure is available. An air pressure gauge gives you that critical real-time information, showing whether there’s enough pressure to safely stop and alerting you to low-pressure conditions before they become dangerous. Without this gauge, you might not know your braking capacity is reduced, which can lead to delayed or insufficient braking.

The other items don’t directly indicate braking readiness: a tachometer shows engine speed, a fuel gauge tracks fuel level, and an odometer records distance traveled. They’re useful for other purposes, but they don’t tell you whether your air brakes have sufficient pressure to operate properly.