CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
You are performing a pre-trip inspection on a combination vehicle's air brake system. After building up full air pressure (120 psi), you shut off the engine, release the parking brake, and fully apply the foot brake. You observe the air pressure gauge. What is the maximum allowable air loss rate for a combination vehicle with the engine off and brakes applied?
- A2 psi in 1 minute
- B3 psi in 1 minute
- C1 psi in 1 minute
- D4 psi in 1 minute
Show answer & explanationAnswer & explanation
Correct answer: D. 4 psi in 1 minute
The applied air brake leakage test checks for leaks in the entire air system, including the service brakes. An air loss rate exceeding 4 psi in one minute for a combination vehicle indicates a problem that must be fixed before driving.
Why the other options are wrong
- A. This is also too low; 2 psi in 1 minute is the maximum allowable for a single vehicle, not a combination.
- B. This is too low; 3 psi in 1 minute is the maximum allowable for a single vehicle with the brakes applied.
- C. This is too low; 1 psi in 1 minute is an ideal, but not required, leakage rate for a combination vehicle.
Applied Air Brake Leakage Test (Combination)
With the engine off and foot brake fully applied, a combination vehicle's air pressure should not drop more than 4 psi in one minute.
- Tests overall air system integrity
- Performed with engine off, brakes applied
- Higher loss indicates leaks, requires repair
Memory trick: Remember, 'Four for more' (combination vehicles) means no more than 4 psi loss.