ASE T4 Truck Brakes Prep Exam Questions & Answers 2026 (21–30)

Question 1

During an air system inspection, the technician notices the purge valve on the air dryer does not release air when the compressor unloads. What is the most likely cause?

TheoryPractice · Accepted Answer

Faulty purge valve. A faulty or stuck purge valve is the most likely cause when no air is released during the compressor unloading cycle. The purge valve should open to release collected moisture and contaminants when the compressor cycles to the unloaded phase.

Question 2

According to FMVSS 121, what is the minimum air pressure build-up time requirement from 85 to 100 psi at engine idle speed?

TheoryPractice · Accepted Answer

45 seconds. FMVSS 121 requires that the air pressure must build from 85 to 100 psi within 45 seconds at engine idle speed. This ensures the vehicle can recover adequate brake system pressure within a reasonable time after multiple brake applications.

Question 3

What could cause a truck's low air pressure warning device to activate at 55 psi instead of the required minimum?

TheoryPractice · Accepted Answer

Improperly adjusted pressure switch. An improperly adjusted pressure switch would cause the low air pressure warning to activate at the wrong pressure. Federal regulations require the low air warning to activate at no less than 60 psi.

Question 4

A truck's air pressure builds to 125 psi and then stops building. Which component should be checked first?

TheoryPractice · Accepted Answer

Governor. The governor should be checked first because it controls the maximum system pressure by regulating when the compressor unloads. If the governor is functioning properly, it should stop the compressor from building pressure once the cut-out pressure (typically 120-135 psi) is reached.

Question 5

A truck experiences excessive air leakage when the service brakes are applied. The most accurate way to determine the exact location of the leak is to:

TheoryPractice · Accepted Answer

Apply soapy water to suspected areas. Using soapy water on suspected areas is the most accurate method to find air leaks. When applied to fittings, valves, or other components, the soapy solution will bubble at the exact point of leakage, allowing precise identification of the problem area.

Question 6

What is the maximum allowable air pressure drop in a truck's service brake system during a static leakage test with brakes applied?

TheoryPractice · Accepted Answer

3 psi per minute. The maximum allowable air pressure drop in a static leakage test with service brakes applied is 3 psi per minute. A greater pressure drop indicates excessive leakage in the brake system that needs to be addressed.

Question 7

A truck's air dryer purge valve releases excessive amounts of oil along with air and moisture. What is the most likely cause?

TheoryPractice · Accepted Answer

Faulty compressor. A faulty compressor with worn piston rings or cylinders is the most likely cause of excessive oil being expelled through the air dryer purge valve. This allows oil to bypass normal sealing and enter the air system where it eventually collects in the air dryer.

Question 8

After replacing several air system components on a truck, what is the proper procedure to check for leaks in the system?

TheoryPractice · Accepted Answer

Build system to full pressure, turn off engine, release and apply brakes, and monitor pressure loss. The proper procedure is to build the system to full pressure, turn off the engine, release and apply the brakes, and monitor pressure loss over time. This tests both the static system (parked with no brakes applied) and the dynamic system (with service brakes applied) for leaks.

Question 9

The supply pressure gauge on a truck consistently shows a lower reading than the actual system pressure. What should be checked first?

TheoryPractice · Accepted Answer

Air pressure gauge. The air pressure gauge should be checked first when it displays inaccurate readings. Gauge malfunction can result in incorrect pressure readings, potentially leading to improper operation based on false information about the system's condition.

Question 10

What is the function of the one-way check valve between the compressor and the first (supply) reservoir?

TheoryPractice · Accepted Answer

Prevent air from flowing back to the compressor from the reservoir. The function of the one-way check valve is to prevent air from flowing back to the compressor from the reservoir. This ensures that compressed air stays in the tank when the compressor is in the unloaded phase or if there's a compressor failure.

ASE T4 Truck Brakes Prep Exam Questions & Answers 2026 (21–30)

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