During normal driving, what keeps the spring brakes released?
- Air pressure ✓
- Spring pressure
- Engine torque
- Bolts
Answer: A. Air pressure
Air pressure keeps the spring brakes released during normal driving.
Why this is the answer
The handbook says spring brakes are meant for parking or for an emergency when service brakes fail. They come on by themselves when system air pressure falls below a set level. That tells you the springs are what apply the brakes, and normal air pressure is what holds them off while you drive. The starting checks also call for enough air pressure before you move, which fits the same idea.
Why the other answers are wrong
- B. Spring pressure — Spring pressure is what applies the spring brakes when air pressure drops, so it does not keep them released.
- C. Engine torque — The passages do not describe engine torque as part of releasing or holding the spring brakes.
- D. Bolts — Bolts are not described in the passages as controlling the spring brakes, so they are not what holds them released.
Remember it
Think of it this way: air holds the springs back; when air is lost, the springs take over.
Official handbook: Official MTO Truck Handbook, Use of brakes