CDL Air Brakes TestInspecting Air Brake SystemsMedium
A driver is performing a pre-trip inspection on a vehicle equipped with air brakes. After building air pressure to 125 psi, the driver turns off the engine, releases the parking brake, and applies full service brakes. The driver observes the air pressure gauge for one minute. What is the maximum allowable air pressure drop for a combination vehicle during this test?
- A1 psi
- B4 psi
- C2 psi
- D3 psi
Show answer & explanationAnswer & explanation
Correct answer: B. 4 psi
For a combination vehicle (truck and trailer), the maximum allowable air pressure drop with the engine off, parking brake released, and full service brakes applied is 4 psi in one minute. This test assesses the integrity of the service brake system for leaks.
Why the other options are wrong
- A. 1 psi is too low; this is sometimes the maximum for a straight truck with the parking brake set, but not a combination vehicle with service brakes applied.
- C. 2 psi is also too low; this is the maximum for a straight truck with service brakes applied.
- D. 3 psi is incorrect; the standard is 4 psi for combination vehicles.
Air Brake Leak Test (Applied)
A test performed during a pre-trip inspection to check for leaks in the service brake system by applying full brakes and observing pressure drop.
- Engine off, parking brake off, full service brake applied.
- Pressure drop is measured over one minute.
- Different maximum drops for straight trucks vs. combination vehicles.
Memory trick: Combination's four, straight's two; that's the leak test rule for you!