In a dual air brake system, if one system loses air pressure, what will happen?

• A. All brakes fail
• B. Front brakes fail only
• C. Rear brakes fail only
• D. Either front or rear brakes will not be fully operational

Answer: (D)

Dual air brake systems split the air supply into two independent circuits, with each circuit powering different brakes. If one circuit loses air, the brakes connected to that circuit won’t function as well, while the brakes on the other circuit keep working. So you’ll end up with brakes that aren’t fully operational on whichever side (front or rear) that circuit governs. It isn’t all brakes failing, because the other circuit still has pressure and can operate. It also isn’t guaranteed to be only the front or only the rear in every situation—the failure affects the brakes served by the failed circuit, which could be the front set or the rear set depending on the vehicle’s layout.