Air Brake Emergencies and Brake Fading
The Air Brake Emergencies and Brake Fading domain tests what you do when brakes overheat, fade, or lose pressure. The exam wants a safe sequence, not a panic reaction.
Brake fade
- Fade is loss of braking force from overheated drums, shoes, or discs.
- Causes include riding the brakes on a long downgrade, repeated high-speed stops, and poor adjustment.
- Signs include a pedal that feels normal but the vehicle will not slow, smoke, a hot smell, and longer stopping distance.
- Response: downshift, use the engine brake or retarder, pull off safely, and let the brakes cool. Do not spray water on hot drums; they can crack.
Air loss and emergency stopping
- If pressure drops, the low-pressure warning should activate before 60 psi.
- Spring brakes apply automatically at low pressure, commonly 20–45 psi; they are not a normal service brake.
- If service brakes fail, use the emergency brake control if equipped, downshift, and look for an escape ramp or safe runoff area.
- Do not pump the brake pedal hoping to rebuild pressure.
Worked example: On a long descent, the pedal goes to the floor and the truck does not slow. You downshift, use the engine brake, and pull onto an escape ramp. Reasoning: the brakes have faded from heat; continuing downhill will only make it worse, and stopping on the grade may be impossible.
Traps: thinking spring brakes can stop a loaded truck quickly; using the parking brake as a service brake; continuing after a low-pressure warning; opening a drain cock to “test” pressure on a grade.