Using Air Brakes
The Using Air Brakes domain tests smooth control, stopping-distance judgment, and downhill technique. Air brakes are not instant: pressure must travel through lines and chambers before the shoes move.
Core rules
- Apply the service brake progressively and firmly. Do not fan or pump the treadle in normal stopping.
- Brake lag adds distance. At 55 mph, brake lag alone can be about 32 feet.
- Total stopping distance is perception distance + reaction distance + brake lag + braking distance.
- On a downgrade, select a low enough gear before the descent and use the engine brake or retarder if equipped.
- Use brakes firmly enough to slow the vehicle, then release. Do not ride the brakes continuously.
- Watch the air gauge. If pressure falls toward the warning range, stop and find the cause.
Worked example: At 55 mph, approximate distances are 60 feet perception, 60 feet reaction, 32 feet brake lag, and 216 feet braking-about 368 feet total. If you follow another truck at 200 feet, you cannot stop if it stops suddenly, even with perfect brakes.
Traps: treating air brakes like hydraulic brakes with no lag; coasting downhill in neutral; using the parking brake as a service brake; ignoring a pressure drop because the vehicle still slows. Also remember that a light, steady application on a long grade can overheat the brakes faster than short, firm applications.