What is the purpose of the modulating control valve (when equipped)?
Answer: C
To allow gradual application of the spring brakes when the service brakes fail.
When equipped, the modulating control valve allows the driver to gradually apply the spring brakes in the event of a service brake failure. Or if the service brakes have faded to the point that they will benefit from supplemental application of the spring brake.
All choices
- ATo instantly apply the emergency brakes when air pressure is lost to the trailer.
- BTo balance air pressure between the truck and trailer brakes when the brake pedal is pressed.
- CTo allow gradual application of the spring brakes when the service brakes fail.
Why the other answers are wrong
A. To instantly apply the emergency brakes when air pressure is lost to the trailer.
Choice A is wrong because the modulating control valve does not instantly apply emergency brakes but allows gradual spring brake application.
B. To balance air pressure between the truck and trailer brakes when the brake pedal is pressed.
Choice B is wrong because the valve does not balance air pressure between truck and trailer brakes during normal braking.
CDL Air Brakes Test Info
Questions on Exam
25
Passing Score
80%
Time Limit
Typically 30 minutes; varies by testing location
The Air Brakes test content is federally standardized, so this question applies in every state. Age limits, fees, and testing procedures do vary — check your state's CDL requirements.
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More CDL Air Brakes Practice Questions
- The most common type of foundation brake found on heavy vehicles is the:
- Which procedure is used for the spring brake test?
- The air brake lag distance at 55 mph on dry pavement adds about ____ feet.
- The spring brakes of a tractor-trailer will activate once air pressure drops to approximately:
- The spring brakes used on the chambers in a straight truck will bring you to a stop when air pressure drops below ____ psi.