CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
A combination vehicle is equipped with a 'dual air brake system'. During a pre-trip inspection, you notice that the primary (rear axle) air gauge reads 100 psi and the secondary (front axle) air gauge reads 60 psi. What is the correct action to take?
- AContinue driving, as this difference is acceptable for a dual system.
- BDo not drive the vehicle and have the air brake system repaired immediately.
- CPump the brake pedal rapidly to equalize the pressures.
- DUse the trailer hand valve to increase pressure in the secondary system.
Show answer & explanationAnswer & explanation
Correct answer: B. Do not drive the vehicle and have the air brake system repaired immediately.
A dual air brake system uses two separate air systems, each with its own reservoir, for safety. A significant pressure difference (40 psi in this case) between the primary and secondary systems indicates a serious malfunction or leak in one of the systems. Driving with such a discrepancy is unsafe and illegal.
Why the other options are wrong
- A. A difference of 40 psi is not acceptable; it indicates a serious problem.
- C. Pumping the brakes would further deplete air, especially in the system with lower pressure, worsening the situation.
- D. The trailer hand valve controls trailer brakes and would not directly increase pressure in the tractor's secondary system.
Dual Air System Pressure Discrepancy
A dual air brake system has two independent air systems (primary and secondary). A significant and sustained pressure difference between these systems indicates a malfunction, leak, or issue with a check valve, requiring immediate repair.
- Each system operates independently for safety.
- A large pressure difference (e.g., >20-25 psi) is critical.
- Indicates a leak or component failure (e.g., check valve).
- Vehicle should not be driven until repaired.
Memory trick: Dual gauges, not equal, a warning you'll hear, fix it or risk it, with safety held dear.