Braking Systems and Performance Standards
Braking is the most safety-critical system on a heavy vehicle, and exam questions test both component condition and performance. You must understand the components of heavy-vehicle braking: service brakes, parking brake, air systems (compressor, reservoirs, valves, lines), and any trailer connections. Know that air-brake systems must be free of leaks, build pressure correctly, and hold pressure; moisture and oil contamination are defects.
Key concepts include brake balance and the requirement that brakes operate evenly across axles; a dragging or non-functioning brake on one wheel is a defect. Understand the parking brake must hold the vehicle on a gradient. Traps include confusing component inspection with performance testing - both are required. Another trap is assuming a vehicle with 'good pedal feel' passes; air systems can feel normal while leaking.
Rule of thumb: a brake system must both look right and perform right. Worked example: if a reservoir loses pressure noticeably during a leak-down check, that is a defect regardless of how the brakes feel in a static push - the system must hold pressure to be certified. Do not certify a brake system you have not verified for leaks and function.