CDL Air Brakes TestAir Brake System PartsEasy
A driver performs a pre-trip inspection and tests the service brakes. They notice that when the brake pedal is pressed, the air pressure drops by approximately 5 psi, and then quickly recovers when the pedal is released. This process is repeated several times with consistent results. What does this indicate about the air brake system?
- AThe air compressor is failing to build adequate pressure.
- BThe foundation brakes are seized and require excessive pressure.
- CThe system is functioning correctly, demonstrating normal air usage.
- DThere is a significant air leak in the system.
Show answer & explanationAnswer & explanation
Correct answer: C. The system is functioning correctly, demonstrating normal air usage.
A normal air brake system will experience a temporary pressure drop (typically 2-3 psi for a single unit, 3-4 psi for a combination) when the service brakes are applied, as air is used. The compressor then quickly recovers this pressure. A 5 psi drop in a combination vehicle would be normal.
Why the other options are wrong
- A. The quick recovery indicates the compressor is functioning adequately.
- B. Seized foundation brakes would likely prevent movement or cause excessive heat, not just a normal pressure drop.
- D. A significant leak would cause a larger, sustained pressure drop or failure to recover.
Normal Air Brake Operation
During normal service brake application, a small amount of air pressure is consumed, causing a temporary gauge drop, which the air compressor quickly replenishes.
- Minor pressure drop (2-4 psi) is normal upon brake application.
- Air compressor should quickly recover pressure.
- Indicates a healthy, sealed system.
Memory trick: Pressure dips, then Puffs back, a normal brake dance.