Air Brake Test Pre-trip Inspection Procedures 2 — Questions and Answers
Question 1: During a pre-trip air brake inspection, at what pressure should the low air pressure warning signal activate?
- Before air pressure drops below 60 psi (Correct answer)
- At exactly 90 psi
- Only when pressure reaches 0 psi
- At 75 psi during engine-off testing only
Correct answer: Before air pressure drops below 60 psi
The low air pressure warning device must activate before pressure falls below 60 psi, giving the driver time to stop safely before spring brakes engage at approximately 20–45 psi.
Federal regulations require the low air pressure warning to activate before pressure falls below 60 psi. This gives the driver adequate warning to bring the vehicle to a safe, controlled stop before pressure drops into the range where spring brakes will apply automatically (typically 20–45 psi). Confirming this warning works correctly is a required part of the pre-trip inspection.
Question 2: What is the correct procedure to test the spring brakes during a pre-trip inspection?
- Charge the system, chock wheels, release service brakes, pull the parking brake knob, and attempt to move the vehicle (Correct answer)
- Apply service brakes at full pressure and measure stopping distance
- Release all air from the system and verify the vehicle does not roll
- Start the engine and check that the spring brake light extinguishes within 30 seconds
Correct answer: Charge the system, chock wheels, release service brakes, pull the parking brake knob, and attempt to move the vehicle
With wheels chocked, fully charged system, and service brakes released, pulling the parking brake control should firmly prevent any vehicle movement — confirming the spring brakes are functioning.
To test spring brakes during pre-trip, build air to operating pressure and chock the wheels for safety. Release the service brakes, then pull the parking brake control (yellow diamond button). Attempt to drive forward or roll the vehicle. If the spring brakes are functioning correctly, the vehicle should not move. If it moves, the spring brakes are not holding and must be repaired before the vehicle is operated.
Question 3: How do you test the tractor protection valve during a pre-trip inspection?
- Close the shut-off valves on the rear, reduce trailer air pressure to trigger the valve, and confirm it closes (Correct answer)
- Apply service brakes repeatedly until trailer air line pressure drops below 30 psi
- Disconnect the trailer and verify tractor brakes function independently
- Start the engine and observe that trailer supply pressure builds to 100 psi within 60 seconds
Correct answer: Close the shut-off valves on the rear, reduce trailer air pressure to trigger the valve, and confirm it closes
The tractor protection valve is tested by reducing trailer air pressure (by closing shut-off valves and draining air) until it drops to the activation range, confirming the valve closes to protect tractor air.
With the trailer supply charged and engine off, close the trailer's rear shut-off valves to isolate it. Use the trailer hand valve or step on the brake pedal to drain trailer air. When pressure drops to approximately 20–45 psi, the tractor protection valve should close automatically, preventing further air loss from the tractor system. Confirm by checking that tractor air pressure is maintained.
Question 4: What is the acceptable pressure drop rate with service brakes applied during the pre-trip air brake leak test?
- Less than 3 psi per minute for single vehicles; less than 4 psi per minute for combination vehicles (Correct answer)
- Less than 10 psi per minute for any vehicle type
- No pressure drop is acceptable in any vehicle
- Less than 5 psi per minute regardless of vehicle configuration
Correct answer: Less than 3 psi per minute for single vehicles; less than 4 psi per minute for combination vehicles
FMCSA/CVSA standards allow a maximum leak rate of 3 psi/min for singles and 4 psi/min for combinations with service brakes applied; anything above is cause for repair.
During pre-trip, with brakes applied and engine off, pressure should not drop more than 3 psi per minute (single) or 4 psi per minute (combination vehicle). Higher leak rates indicate significant brake system leaks — in air lines, chambers, fittings, or valves — that must be repaired. Drivers should allow pressure to stabilize before starting the timed measurement.
Question 5: When inspecting the air compressor belt during a pre-trip, what should you check for?
- Cracks, fraying, proper tension, and correct alignment (Correct answer)
- Belt color and manufacturer markings
- Belt temperature using a thermal gun
- Only the presence of the belt — condition is assessed only during maintenance
Correct answer: Cracks, fraying, proper tension, and correct alignment
The compressor drive belt must be inspected for cracks, wear, fraying, proper tension, and correct alignment to ensure the compressor will maintain adequate air pressure.
The air compressor is driven by the engine through a belt or direct gear drive. On belt-driven compressors, a worn, cracked, or loose belt can slip under load or snap, disabling the compressor. Once the stored air in the tanks is depleted, spring brakes will apply. Pre-trip inspection of the belt ensures it can reliably drive the compressor throughout the trip.
Question 6: During a pre-trip, you notice the air pressure gauge is fluctuating rapidly and erratically. What could this indicate?
- A faulty gauge, a failing compressor governor, or a significant air leak in the system (Correct answer)
- Normal compressor cycling during warm-up
- The automatic drain valves are functioning correctly
- The ABS system is self-testing
Correct answer: A faulty gauge, a failing compressor governor, or a significant air leak in the system
Rapid erratic gauge fluctuation can indicate a malfunctioning gauge, governor problems causing rapid cycling, or a large leak that makes pressure unstable.
Air pressure should rise steadily and hold within the normal range. Erratic fluctuations suggest a problem: a defective gauge may give false readings; a sticking governor can cause rapid compressor cycling; or a major air leak can cause pressure to swing wildly as air escapes and is replaced. Any of these conditions requires investigation before the vehicle is placed in service.
Air Brake Test Pre-trip Inspection Procedures 2
During a pre-trip air brake inspection, at what pressure should the low air pressure warning signal activate?