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CDL · General

What are spring brakes, and how do they work?

Correct answer

Emergency/parking brakes held in the 'off' position by air pressure; if air pressure is lost, the springs apply the brakes automatically (a fail-safe)

  1. A Brakes made of springs that bounce
  2. B Emergency/parking brakes held in the 'off' position by air pressure; if air pressure is lost, the springs apply the brakes automatically (a fail-safe)
  3. C Brakes that only work in spring weather
  4. D Brakes that increase speed

Why this is the answer

SPRING BRAKES are the emergency and parking brakes. They are held in the 'OFF' (released) position by AIR PRESSURE; powerful springs are compressed by the air. If air pressure is LOST (a leak or failure), the springs are released and AUTOMATICALLY APPLY the brakes — a FAIL-SAFE design. CDL air brakes. This is why losing air causes the brakes to apply (stopping the vehicle) rather than fail. Spring brakes typically apply when air pressure falls to around 20-45 psi. For PARKING, you apply the spring brakes (pull the brake knob). Knowing spring brakes are air-held-off, spring-applied (fail-safe) is critical air brake knowledge.
Source: FMCSA CDL Manual — Air Brakes, Spring Brakes

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