Hydraulic brakes or air brakes should not be fanned except...?

The correct choice highlights an important safety consideration when operating vehicles equipped with hydraulic or air brakes. Fanning the brakes, which involves repeatedly applying and releasing the brakes, is typically discouraged as it can lead to brake overheating and decreased effectiveness. However, on slippery pavement, fanning the brakes can help to prevent the vehicle from skidding, providing better control. When drivers apply the brakes intermittently on slippery surfaces, it helps to maintain traction while still managing the vehicle's speed.

In contrast, on long downhill grades, fanning the brakes can actually lead to brake failure due to overheating, making it a hazardous practice. Similarly, during hot weather or on dry pavement, continuous braking without engaging other methods of speed control can also lead to overheating and reduced braking efficiency, potentially resulting in dangerous situations. Hence, the context of maintaining control on slippery conditions makes the first option the most valid scenario for fanning brakes.