ASE T4 Truck Brakes Prep practice questions and answers 2026. Tap an option to test yourself — you'll see the correct answer and a plain-English explanation for every question. Free, no login.
Q11On a combination vehicle, a technician tests the tractor protection valve. When the trailer air supply knob is pulled out, what should happen?
✓ Correct answer: B. Trailer spring brakes should apply automatically
When the trailer air supply knob is pulled out, it should exhaust air from the trailer service and supply lines, causing the trailer spring brakes to apply automatically. This is a safety feature that protects the tractor air system if the trailer breaks away or develops severe air leaks.
Q12A truck's parking brakes apply too slowly when the control valve is activated. Technician A says a restricted exhaust port in the control valve could be the cause. Technician B says that weak power springs in the spring brake chambers could be the cause. Who is right?
✓ Correct answer: A. A only
Only Technician A is correct. Spring brakes apply when air is exhausted from the spring brake chambers. A restricted exhaust port in the control valve would slow down the release of air, causing slow application. Weak power springs (Technician B) would cause reduced holding force, not slow application.
Q13According to DOT regulations, what percentage of a truck's service brake capacity must the parking brake system be able to hold?
✓ Correct answer: D. At least 28%
DOT regulations specify that a vehicle's parking brake system must be able to hold at least 28% of the vehicle's service brake capacity. This ensures that the parking brake has sufficient holding power for the vehicle when parked.
Q14Which component provides the mechanical force in a spring brake chamber for parking brake application?
✓ Correct answer: C. The power spring
The power spring provides the mechanical force for parking brake application in a spring brake chamber. When air pressure is removed from the spring brake chamber, this powerful spring extends to mechanically apply the brakes through the push rod.
Q15The spring brakes on a tractor-trailer do not apply automatically when system air pressure drops below 45 psi. What is the most likely cause?
✓ Correct answer: B. A defective dash control valve
The dash control valve is responsible for monitoring system air pressure and automatically exhausting air from the spring brake chambers when pressure drops below the threshold (typically 40-60 psi). A defective valve would prevent this automatic safety application.
Q16A truck's air compressor is constantly cycling on and off rapidly. What is the most likely cause?
✓ Correct answer: A. Faulty governor
A faulty governor is the most likely cause of rapid cycling. The governor controls when the compressor loads and unloads based on system pressure. If it's malfunctioning, it may signal the compressor to cycle on and off too frequently instead of maintaining proper cut-in and cut-out pressures.
Q17Technician A says that moisture in the air brake system can cause brake fade during repeated stops. Technician B says that moisture in the air brake system can freeze in cold weather and block air flow. Who is right?
✓ Correct answer: D. Both A and B
Both technicians are correct. Moisture in the air brake system can cause brake fade during repeated stops due to decreased friction when water is present. It can also freeze in cold weather conditions, potentially blocking air flow through lines and valves.
Q18A truck's air system takes too long to build pressure. Technician A says a clogged air inlet filter could be the cause. Technician B says a leaking compressor discharge line could be the cause. Who is right?
✓ Correct answer: C. Both A and B
Both technicians are correct. A clogged air inlet filter restricts airflow into the compressor, reducing its efficiency and causing slow pressure build-up. Similarly, a leaking compressor discharge line would allow compressed air to escape, resulting in a slower pressure build in the system.
Q19What component should be inspected first when diagnosing excessive oil in a truck's air system?
✓ Correct answer: B. Compressor
The compressor should be inspected first when diagnosing excessive oil in the air system. A worn compressor with excessive piston ring or cylinder wear can allow oil to bypass into the compressed air, contaminating the entire system.
Q20When checking an air dryer on a truck, what indicates that the desiccant material needs to be replaced?
✓ Correct answer: A. Excessive moisture in the air tanks
Excessive moisture in the air tanks indicates that the desiccant material in the air dryer has lost its effectiveness and needs replacement. The desiccant's primary function is to remove moisture from compressed air before it enters the reservoirs.
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Theory