Air Brake System Malfunctions and Emergencies
Low pressure and failures
This domain tests what happens when air pressure is lost and how to respond. The first warning is the low-pressure buzzer and light, required by 60 psi. If pressure keeps falling, spring brakes apply automatically around 20–45 psi. The vehicle will stop, but not necessarily where you want, so act before that point.
- On low-pressure warning, stop safely, set the parking brake, and do not drive until the system is repaired. Do not fan the brakes to “build up” pressure.
- If service brakes fail, downshift to use engine braking, apply the parking brake gradually, and look for an escape ramp or safe runoff. Sudden full parking-brake application can cause a jackknife.
- A compressor or governor failure shows as pressure that will not build or keeps rising. A ruptured line or chamber can cause a rapid loss; stop immediately.
- Frozen air lines are prevented with an air dryer or alcohol evaporator. Never use an open flame to thaw lines.
- ABS malfunction: the yellow ABS light means ABS is disabled, but base brakes remain. Drive cautiously and get it repaired.
Worked example
A driver sees the low-pressure light and hears the buzzer at 55 psi on a freeway. The correct action is to signal, move to the shoulder, and stop while service brakes still have pressure. Continuing to the next exit risks spring brakes applying in traffic. After stopping, the parking brake will hold the vehicle; the driver should not release it and continue without finding the leak.
Traps
Ignoring the warning because the pedal still feels firm is a fatal error. Pumping the pedal during a leak speeds pressure loss. Using the spring brake as a service brake at highway speed can cause loss of control. Assuming an ABS light means no brakes leads to unnecessary panic.