The spring brakes of a tractor-trailer will activate once air pressure drops to approximately:
Answer: B
20 to 45 psi.
First, the low-air pressure signal will be activated between 55 and 75 psi. Second, as air pressure continues to drop, the tractor protection valve will activate close to 45 psi. Third, the spring brakes will also begin applying at 45 psi as the spring pressure overcomes the remaining air pressure and will be fully applied when the air pressure gets down to 20 psi.
All choices
- A70 to 85 psi.
- B20 to 45 psi.
- C5 to 15 psi.
Why the other answers are wrong
A. 70 to 85 psi.
Choice A is wrong because spring brakes begin applying below 45 psi, not as high as 70 to 85 psi.
C. 5 to 15 psi.
Choice C is wrong because spring brakes are fully applied by 20 psi but start applying at higher pressure near 45 psi.
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
- Characteristics of dual air brake systems include:
- Which component of the air brake system cuts-in at 100 psi to allow the air compressor to run?
- The driver must be able to see a low air pressure warning which comes on before pressure in the service air tanks fall below ____ psi.
- The safety valve is set automatically when pressure is:
- Parking or emergency brakes of trucks and buses can be legally held on by ____ pressure.