Air Brakes and Braking Systems on Combination Vehicles
Air brake questions for combination vehicles test the tractor-trailer air system, protection valves, brake tests, and braking technique on grades. You must know how the trailer gets air and what happens if it separates.
System basics
- The tractor supplies air through the red emergency line; the blue service line signals the trailer brakes.
- The tractor protection valve closes if the trailer breaks away, limiting air loss; trailer spring brakes then apply.
- Trailer spring brakes are for parking and emergency use; service brakes are for normal stopping.
Tests
- Build air to governor cut-out; check leak rate with engine off and brakes applied.
- Test the low-pressure warning and spring brake pop-out.
- After coupling, charge the trailer and test trailer brakes and lights.
- Disconnect the emergency line to confirm trailer spring brakes apply.
Braking technique
- Brake early and progressively; do not fan the pedal.
- On a long downgrade, select a lower gear before descending and use engine braking.
- Use the trailer hand valve only to test trailer brakes, not for parking or normal stopping.
- With ABS, maintain firm pressure; do not pump.
Worked example
A heavily loaded tractor-trailer starts down a long grade in high gear. The driver rides the brakes and ignores the air gauge. Brake fade and low air pressure follow. The correct plan is to downshift before the grade, use engine braking, make short brake applications, and check the gauge.
Traps
- Assuming the trailer is charged just because the lines are connected.
- Using the trailer hand valve as a parking brake.
- Missing the low-pressure warning test.
- Riding the brakes instead of controlling speed with gearing.