During normal driving, what keeps the spring brakes released?
- Engine torque
- Spring pressure
- Bolts
- Air pressure ✓
Answer: D. Air pressure
Air pressure keeps the spring brakes released during normal driving.
Why this is the answer
The handbook says air pressure in the spring parking brake chamber holds the spring parking brake off. It also explains that pushing the parking brake control valve in lets air pressure flow to the spring parking brake chambers, causing the brakes to release. In the released diagram, air at reservoir pressure inflates the parking brake diaphragm and compresses the main spring. So the brakes stay released because air pressure remains in that chamber.
Why the other answers are wrong
- A. Engine torque — Engine torque is not described in the passages as holding the spring brakes released; release is tied to air pressure in the spring parking brake chamber.
- B. Spring pressure — Spring pressure is what the air pressure has to overcome, and when air is exhausted the spring applies the brakes rather than keeping them released.
- C. Bolts — The passages do not mention bolts holding the spring brakes released; they describe air pressure inflating the diaphragm and compressing the main spring.
Remember it
Think of air pressure as the force holding the spring back so the parking brake stays off.