Air Brakes and Brake System Checks on Combination Vehicles
What the check tests
Combination vehicles have a service line (blue) and an emergency line (red). The tractor protection valve closes the tractor's air supply if trailer pressure falls, and the trailer supply valve controls the emergency line. The exam tests the standard air-brake sequence with combination-specific limits.
The sequence
- Build air: at 1,000–1,200 rpm the compressor should raise pressure from 85 to 100 psi within 45 seconds.
- Static leak test: engine off, parking brakes released, fully charged, then watch the gauge. Loss should be no more than 3 psi per minute for a combination (2 psi for a single vehicle).
- Applied leak test: hold the foot brake fully applied for one minute. Loss should be no more than 4 psi per minute for a combination (3 psi for a single).
- Low-pressure warning: fan the brakes; the warning must come on before pressure drops below 60 psi.
- Spring-brake test: keep fanning; the tractor and trailer protection valves should pop out between 20 and 45 psi, applying the spring brakes.
- Check that the trailer brakes apply and release with the hand valve or foot brake.
Worked example
Gauge reads 120 psi. Engine off, brakes released; after two minutes it reads 113 psi. Drop = 7 psi in 2 minutes = 3.5 psi/min, which exceeds the 3 psi/min combination limit, so the vehicle fails the static test. Look for a leak at the gladhands, trailer valves, or lines.
Traps
Leaving the engine running during the leak test; using the 2 psi single-vehicle limit on a combination; confusing static and applied limits; assuming the warning works because the buzzer sounds at 30 psi.