Air Brake System Failures and Emergencies
Recognizing trouble
Failures show up as sound, feel, or gauge movement. Audible leaks (hiss at a chamber, hose, or valve), a gauge that keeps dropping, a low-pressure warning, a spring brake that applies by itself, or a pedal that feels spongy or goes to the floor all demand action. A compressor failure or broken belt means pressure will only fall; a failed governor can overpressure the system and lift the safety valve.
What to do
- Stay calm and keep control. Grip the wheel, keep the vehicle straight.
- Downshift to use engine braking and, if equipped, the retarder.
- Apply the brakes progressively - you may have only a limited number of applications left.
- Move off the road as soon as you can do so safely and stop.
- Set the parking brake. If air is gone, the spring brakes should already have applied; if not, use the knob and chock the wheels.
- Do not continue until the fault is found and repaired.
Emergency stopping
If service brakes fail completely, the spring brakes will apply automatically as pressure bleeds off - that is the emergency function. You can also use the parking brake control gradually to modulate spring brake application, but do not apply it suddenly at speed or you may lock the wheels and lose control. Use run-away ramps, guardrails, or a soft shoulder as a last resort, and sound the horn to warn others.
Traps and a rule of thumb
Questions test whether you know the spring brake is the emergency brake, not the service brake, and that it applies on loss of air. Another trap: pulling the parking knob hard at highway speed is dangerous, not a smart emergency move. Also remember that a dual-system failure of one circuit still leaves the other circuit working - brake gently and get stopped.
Rule of thumb: when the warning sounds, you have a short window - slow down, get off the road, stop, and stay stopped.