Air Brake System Malfunctions and Emergencies
What the exam tests
Malfunction questions ask you to recognize the symptom, explain the cause, and choose a safe emergency response.
Common malfunctions
- Brake fade: heat from overuse reduces friction; the pedal may feel normal but stopping power drops. Use lower gears and short applications.
- Brake lag: air brakes take time to apply; at 55 mph the vehicle can travel about 32 feet before the brakes take hold. Increase following distance.
- Air leak: pressure falls steadily; the low-pressure warning sounds. Stop and repair.
- Compressor failure: pressure will not build or builds too slowly; the governor may not cut in.
- Frozen air lines: moisture freezes in cold weather; use an air dryer or alcohol evaporator and drain tanks.
- Contaminated air: oil or water in tanks can damage valves and cause sticking brakes; drain tanks regularly.
- Dragging or grabbing brakes: misadjusted slack adjusters, weak return springs, or contaminated linings.
Worked example
The service brake pedal goes to the floor and the vehicle will not slow normally, but the gauge still shows 100 psi. Reasoning: this is a service brake failure, not necessarily an air loss. The driver should downshift, use the engine retarder, and apply the parking/spring brake gradually. Slamming the yellow knob at highway speed can lock the wheels. If braking is still insufficient, use an escape ramp.
Traps that cost marks
- Applying the spring brake suddenly at high speed; it can cause a skid.
- Assuming a full gauge means the service brakes are working.
- Ignoring brake fade and continuing to ride the pedal.
- Treating a compressor failure as a minor issue; pressure will eventually run out.
- Forgetting that water in the tanks can freeze and block lines.