Normal Driving with Air Brakes
This domain tests braking technique, the causes of brake fade, and how speed, weight, and grade change stopping distance. Montana's long mountain grades make this section practically important and frequently tested.
Technique
- Use controlled braking: apply the pedal as hard as you can without locking wheels, and hold steering.
- Do not fan the brakes for normal stops - repeated on-off application on a downgrade builds heat without giving the drums time to cool.
- Brake in short, firm applications, then release and let the drums cool while the engine brake and gearing hold the speed.
- Choose the gear before the descent, not partway down. If you are braking hard to hold speed, you are in too high a gear.
- Brake lag exists: air takes time to travel to the chambers, so total stopping distance = perception + reaction + lag + effective braking.
Brake fade
- Fade is caused by excessive heat: drums expand and linings glaze, so the same pedal force produces less stopping power.
- Common causes: riding the brakes downhill, repeated high-speed stops, an overloaded or out-of-adjustment brake.
- The cure is prevention - lower gear, less speed, cooler drums - not more pedal.
Worked example
A loaded tractor-trailer enters a 6% grade at 55 mph in high gear. The driver brakes continuously and within a mile the pedal feels normal but the truck will not slow. Reasoning: normal pedal feel with poor retardation in a descent is heat fade, not an air leak. The correct response is to downshift progressively, brake hard and briefly when needed, and get to a lower speed - the earlier the better.
Traps that cost marks
Thinking fade means the brakes are wet or out of adjustment; believing heavier loads shorten stopping distance; using the parking brake as a retarding device; assuming engine braking is safe on ice.