The effectiveness of the spring brakes for emergencies or parking
- can only be tested by brake service specialists.
- increases as heat increases in the brakes.
- is not affected by the adjustment of the service brakes.
- depends on the adjustment of the service brakes. ✓
Answer: D. depends on the adjustment of the service brakes.
Spring brake effectiveness depends on the adjustment of the service brakes.
Why this is the answer
The handbook says a spring brake's ability to stop or hold the vehicle depends on brake condition and correct adjustment. If the brakes are out of adjustment, the spring brakes may not stop the vehicle or keep it stationary. The same source notes that poor adjustment weakens both service and spring braking, making adjustment the deciding factor among the choices.
Why the other answers are wrong
- A. can only be tested by brake service specialists. — The handbook describes a road check: hold the spring brakes and gently apply engine power to see whether the vehicle stays put, so specialist-only testing is not what it says.
- B. increases as heat increases in the brakes. — No passage links spring brake effectiveness to heat; the stated factors are brake condition and adjustment.
- C. is not affected by the adjustment of the service brakes. — The handbook says the opposite: spring brake performance is reduced when the service brakes are poorly adjusted.
Remember it
A spring brake can only hold as well as the brakes are adjusted.