Inspection and Air Brake Checks
Air brake checks on a combination vehicle confirm that the tractor and trailer brake systems work together. Candidates must know the standard tests: the static leak test (build pressure, shut off the engine, and watch for pressure loss), the applied leak test (hold the brake pedal and watch the gauge), the low-air warning test (the warning must come on before pressure drops too low), the spring brake test (with low air, the parking brake should apply), and the tractor protection valve check. Know that the tractor protection valve closes to keep air from escaping if the trailer separates, and that the trailer spring brakes apply when the emergency line loses pressure. Traps: mixing up which line does what, believing the trailer brakes apply with the service line only, or thinking the low-air warning is optional. Another trap is testing on a slope or without chocking. Rule of thumb: test in order - build pressure, check for leaks, check the warning, check the spring brakes, then check the tractor protection valve. Worked example: if pressure drops faster than the allowed rate during the static test, there is a leak and the vehicle should not be driven until it is fixed.