CDL Air Brakes TestDual Air Brake SystemsMedium
A driver notices that when the service brakes are applied, the air pressure in both the primary and secondary tanks drops by approximately 5 psi. This occurs consistently with each firm brake application. What does this indicate about the vehicle's air consumption?
- AThis is normal air consumption for a combination vehicle.
- BThere is a significant leak in the service brake lines.
- CThe air compressor is failing to keep up with demand.
- DThis indicates excessive air consumption for a straight truck.
Show answer & explanationAnswer & explanation
Correct answer: A. This is normal air consumption for a combination vehicle.
For a combination vehicle, a drop of 5 psi or less per firm brake application is generally considered normal. This consumption accounts for the air used to activate the brake chambers on both the tractor and trailer.
Why the other options are wrong
- B. A significant leak would cause pressure to drop even without brake application, or cause a much larger, continuous drop during application.
- C. If the compressor was failing, the pressure would continue to drop more severely or not recover, rather than consistently dropping 5 psi and recovering.
- D. This would be excessive for a straight truck, where the normal drop is usually less than 3 psi.
Normal Air Consumption (Combination Vehicle)
For a combination vehicle, a normal drop in air pressure during a firm service brake application is typically 5 psi or less.
- Applies to combination vehicles
- Drop per firm application
- Indicates healthy system
Memory trick: Five for a 'combination' is fine, but less for a 'straight' line.