Next you should fan the brakes down to make sure all air brake safety devices are working, but what should you do before starting to fan the brakes?

You will need to have the key on to accessory or to the position right before you start the vehicle. This needs to be done so that the alarm and light for the low air warning can come on alerting of low air pressure. If the light and alarm does not come on when you are taking your test you may automatically fail.