CDL Air Brakes TestDual Air Brake SystemsHard
A Class A CDL driver is performing a pre-trip inspection on a tractor-trailer equipped with a dual air brake system. The driver notes that the primary air gauge shows 110 psi and the secondary air gauge shows 90 psi. What action should the driver take before operating the vehicle?
- APump the brake pedal to equalize the pressure between the two tanks.
- BProceed with the trip, as these pressures are within acceptable operating limits.
- CIdentify and repair the cause of the 20 psi pressure differential.
- DContinue to build air pressure until both tanks reach 125 psi.
Show answer & explanationAnswer & explanation
Correct answer: C. Identify and repair the cause of the 20 psi pressure differential.
While both pressures are above the minimum operating threshold (60 psi), a significant difference between the primary and secondary system pressures (20 psi in this case) indicates a potential problem such as a leak or a faulty check valve. This differential needs to be investigated and corrected before driving.
Why the other options are wrong
- A. Pumping the brake pedal would reduce pressure, not equalize a differential caused by a leak or faulty component.
- B. Although above 60 psi, a 20 psi differential is a sign of an issue that needs attention.
- D. Building to 125 psi might temporarily mask the issue or indicate a governor malfunction, but it doesn't address the pressure differential itself.
Dual Air System Pressure Differential
In a dual air brake system, significant and persistent pressure differences between the primary and secondary air tanks can indicate a problem.
- Gauges should show similar pressures.
- A difference of 20-25 psi or more may indicate a leak or faulty component.
- Requires investigation and repair before operation.
Memory trick: Two gauges, two numbers, if unequal, something's troubled.