Air Brake Failure Modes: Types and Causes
Air brake systems can fail in several distinct patterns, each requiring different responses. Complete air pressure loss (catastrophic failure): caused by a major air line rupture, compressor failure, or large air leak. The low pressure warning (buzzer and/or warning light at approximately 60 PSI) activates, followed rapidly by spring brake application when pressure drops to 20-45 PSI (spring set pressure varies by vehicle). If this occurs at speed, the driver has very limited time — seconds — between the warning and spring brake application. Partial air pressure loss: a slower leak or minor air line damage that gradually reduces system pressure over time. Warning light activates as pressure drops below 60 PSI. The driver has more time to respond safely but must act immediately — pull off the highway, alert dispatch, and do not attempt to continue. Gradual brake fade: not an air pressure issue, but overheating of brake friction material from extended or severe service braking (mountain descents). Pedal feel may not change (air brakes maintain pedal travel through the air circuit), but stopping distances increase. Warning: braking effectiveness decreasing despite normal pedal response indicates thermal fade — stop and allow brakes to cool completely before proceeding. Single-circuit vs. dual-circuit failure: modern air brake systems are dual circuit (primary and secondary) — a failure in one circuit reduces but does not eliminate braking. The rear brakes may use the secondary circuit while fronts use the primary, or vice versa. Single-circuit failure leaves approximately 50-60% of normal braking capacity.