CDL General Knowledge TestAir Brakes (if applicable)Medium
A driver is performing a pre-trip inspection on a vehicle equipped with air brakes. They notice that when the brake pedal is fully depressed, the air pressure gauge drops from 120 psi to 80 psi. What is the most likely cause of this significant pressure drop?
- AThere is excessive air loss due to leaks in the system.
- BThe slack adjusters are properly adjusted.
- CThe air compressor is operating normally.
- DThe low air pressure warning light is malfunctioning.
Show answer & explanationAnswer & explanation
Correct answer: A. There is excessive air loss due to leaks in the system.
A significant drop in air pressure (more than 20-30 psi) when the brakes are applied indicates excessive air loss, usually due to leaks in the brake system. This is a critical safety issue that needs immediate attention.
Why the other options are wrong
- B. Properly adjusted slack adjusters would not cause a large pressure drop; maladjusted ones could indirectly contribute to poor braking but not direct pressure loss.
- C. Normal compressor operation would maintain pressure or recover quickly, not show a large drop upon brake application.
- D. While a malfunctioning warning light is possible, it wouldn't explain the actual pressure drop observed on the gauge.
Excessive Air Loss
A condition in an air brake system where too much air escapes, often indicated by a large pressure drop when the service brakes are applied.
- Indicates leaks in the system.
- Can lead to brake failure.
- Requires immediate repair.
Memory trick: Pressure drops fast, leaks are vast.