CDL Air Brakes TestDual Air Brake SystemsMedium
A driver is performing an air brake check on a vehicle with a dual air system. After ensuring the air pressure is at maximum, they turn off the engine and release the parking brake. They then apply firm pressure to the service brake pedal. What is the maximum allowable air pressure drop in one minute for this vehicle combination (tractor-trailer)?
- A2 psi
- B5 psi
- C3 psi
- D4 psi
Show answer & explanationAnswer & explanation
Correct answer: D. 4 psi
For a combination vehicle (tractor-trailer), when the engine is off and the service brakes are applied, the maximum allowable air pressure drop is 4 psi in one minute. This applied air loss test checks for leaks under braking conditions.
Why the other options are wrong
- A. 2 psi is the maximum static loss for a single unit vehicle, not an applied loss for a combination.
- B. 5 psi is generally considered excessive and indicates a leak that needs repair.
- C. 3 psi is the maximum applied loss for a single unit vehicle, or static loss for a combination vehicle, but not applied for a combination.
Applied Air Loss Test (Combination Vehicle)
This test checks for air leaks when the service brakes are applied, with the engine off, for a combination vehicle.
- Engine off, parking brake released, service brake applied.
- Maximum allowable drop is 4 psi in 1 minute.
- Tests the system under load conditions.
Memory trick: Four for a combo, when you press the pedal to the floor, no more.