CDL Air Brakes TestInspecting Air Brake SystemsMedium
During a pre-trip inspection, a driver performs a static air leak test (engine off, parking brake applied). After the initial pressure drop, the air pressure gauge shows a steady 120 psi. After one minute, the pressure has dropped to 117 psi. The vehicle is a single unit truck. What does this indicate?
- AA normal and acceptable air loss.
- BAn excessive air leak requiring immediate repair.
- CA faulty parking brake system.
- DInsufficient air in the primary reservoir.
Show answer & explanationAnswer & explanation
Correct answer: B. An excessive air leak requiring immediate repair.
For a single unit truck during a static air leak test (engine off, parking brake applied), the air pressure should not drop more than 2 psi in one minute. A 3 psi drop (120 - 117 = 3) is excessive and indicates a leak that must be repaired.
Why the other options are wrong
- A. Normal static air loss for a single unit is no more than 2 psi in one minute.
- C. While the parking brake holds the vehicle, the leak test checks the entire system, not just the parking brake function.
- D. The initial pressure of 120 psi is sufficient; the issue is the rate of pressure loss.
Static Air Leak Test (Single Unit)
This test checks for air leaks in the air brake system with the engine off and parking brake applied, without operating the service brakes.
- Engine off, parking brake applied.
- Fully charge air system.
- After initial stabilization, monitor pressure for one minute.
- Pressure drop should not exceed 2 psi for a single unit truck.
Memory trick: Static, parked, a 2 psi drop is all you're allowed to flop.