Final answer:
Hydraulic or air brakes should not be fanned, except on long downhill grades to avoid overheating and maintain control.
Step-by-step explanation:
Hydraulic brakes or air brakes should not be fanned except on long downhill grades. Fanning the brakes means pumping them instead of applying steady pressure. This is generally discouraged because it can lead to a loss of braking efficiency or even brake failure. However, on long downhill grades, it can be necessary to fan the brakes to prevent them from overheating and maintaining control of the vehicle.