You must make heavy brake applications as part of your pre-trip inspection. Why should you release the spring brakes first?
- To obtain a true gauge reading
- To permit the brakes to apply fully
- To prevent the brakes from applying fully
- To avoid compounding the pressures ✓
Answer: D. To avoid compounding the pressures
Release the spring brakes first to avoid compounding the pressures.
Why this is the answer
The handbook warns that spring parking brakes must be released before you make heavy service brake applications, such as during a pre-trip inspection. It calls this avoiding compounding the brakes. When the spring brake is applied, spring force pushes the brake mechanism to apply the brakes. If you then make a heavy service brake application without releasing the spring brakes, the service brake force is added to the spring brake force. Releasing the spring parking brakes first keeps those two applying forces from being combined.
Why the other answers are wrong
- A. To obtain a true gauge reading — The passages tie releasing the spring brakes to avoiding compounding, not to getting a true gauge reading.
- B. To permit the brakes to apply fully — The spring brakes already apply the brakes, so releasing them is not what allows the brakes to apply fully.
- C. To prevent the brakes from applying fully — Releasing the spring brakes does not prevent the service brakes from applying fully; it prevents spring and service brake force from being compounded.
Remember it
Think spring force plus service force equals compounding, so release the spring brakes before a heavy brake application.