Air brake equipped vehicles must have:

• A. A parking brake control
• B. A supply pressure gauge
• C. A tachometer
• D. A fuel gauge

Answer: (B)

Monitoring the air brake system pressure is essential because air brakes rely on stored compressed air to operate. The supply pressure gauge shows how much pressure is available in the system, so you can verify there’s enough pressure to release and apply the brakes and to keep the vehicle under control. Without this gauge, you wouldn’t know whether the brakes will function properly, which is unsafe. A tachometer measures engine RPM and isn’t required for braking, and a fuel gauge, while useful, has no direct bearing on brake operation. Parking brake controls are common, but the requirement highlighted here is the supply pressure gauge, since it directly indicates the readiness of the braking system.