A driver notices that when they apply the service brakes, the air pressure in both the primary and secondary systems drops by approximately 15 psi, and then quickly recovers when the pedal is released. The vehicle is a combination vehicle. What is this most indicative of?
- ANormal operation of the dual air brake system with proper air consumption.
- BA major leak in the service brake lines.
- CInsufficient air storage capacity in the tanks.
- DA faulty air compressor governor.
Show answer & explanationAnswer & explanation
Correct answer: A. Normal operation of the dual air brake system with proper air consumption.
When the service brakes are applied, air is consumed from both the primary and secondary tanks to operate the brake chambers. A drop of 15 psi in a combination vehicle is within the normal range for air usage during a firm brake application. The quick recovery upon release indicates the compressor is functioning correctly and recharging the tanks. A major leak would cause a continuous, non-recovering drop. A faulty governor would affect cut-in/cut-out pressures. Insufficient capacity would lead to a much larger drop and slower recovery.
Why the other options are wrong
- B. A major leak would cause a continuous drop, potentially to spring brake engagement, and slow recovery.
- C. Insufficient capacity would result in a much larger pressure drop per brake application and potentially slower recovery.
- D. A faulty governor would lead to incorrect compressor cut-in/cut-out, not this specific pressure drop pattern.
Normal Air Consumption (Combination)
The expected temporary drop in air pressure (typically 10-25 psi) in a dual air brake system during a full service brake application in a combination vehicle, followed by recovery when the brakes are released, indicating normal system operation.
- Normal for combination vehicles.
- Indicates air is being used by brake chambers.
- Recovery confirms compressor function.
Memory trick: Press the pedal, air goes down, release, air comes back up.