When descending a long, steep mountain grade in a heavy commercial truck, how should auxiliary braking devices (such as the engine exhaust brake / 排气制动 or hydraulic retarder / 液力缓速器) be utilized?

1. Engage the auxiliary brake in conjunction with a matching low transmission gear to maintain a steady safe speed, minimizing service brake use
2. Shift the transmission into neutral and rely solely on the auxiliary exhaust brake
3. Use the service foot brake continuously while leaving the auxiliary brake turned off
4. Engage the auxiliary brake only after the service brakes begin smoking and fading