CDL Combination Vehicles TestCombination Vehicle Air BrakesEasy
A driver is performing a pre-trip inspection of a combination vehicle's air brake system. They notice that the air pressure in the service tanks is steady at 125 psi, but when the foot brake is applied, the pressure drops to 100 psi and holds. Which of the following is the most likely cause of this pressure drop?
- AA faulty air compressor governor.
- BNormal operation of the air brake system.
- CA significant leak in the glad hand connections.
- DThe spring brakes are partially engaged.
Show answer & explanationAnswer & explanation
Correct answer: B. Normal operation of the air brake system.
This pressure drop is a normal part of the air brake system's operation. When the foot brake is applied, air is consumed to actuate the brake chambers, causing a temporary, expected pressure drop.
Why the other options are wrong
- A. A faulty governor would affect the cut-in/cut-out pressures, not cause a temporary drop upon brake application.
- C. A significant leak would cause a continuous pressure drop, not just a temporary one upon application.
- D. Spring brakes engage when air pressure is lost, not when the foot brake is applied at normal operating pressures.
Normal Air Pressure Drop on Brake Application
When the foot brake is applied in an air brake system, a temporary drop in air pressure is expected as air moves from the reservoirs to the brake chambers to apply the brakes.
- This is a normal operational characteristic.
- The pressure should then stabilize.
- Excessive or continuous drop indicates a leak.
Memory trick: Air power for brakes means pressure will bow, but then it will hold, steady and slow.