Which statement is true regarding the spring brake in an air brake system?

• A. It engages only when the engine is running
• B. It engages when PSI falls within a specific range
• C. It is independent of system pressure
• D. It is used to boost front brakes only

Answer: (B)

In air brake systems, the spring brake acts as a fail-safe that automatically applies when system pressure falls below a certain level. The air pressure holds the spring brake released; when the pressure drops into the low range (below the hold-off threshold), the spring forces the brakes on for parking or emergency protection. This behavior is independent of whether the engine is running, and it applies to the brakes controlled by that chamber, not just the front brakes. So the statement that the spring brake engages when the PSI drops to a specific low range best describes how it works.