310T Air Brake Systems Diagnosis Questions and Answers 1 — Questions and Answers
Question 1: A technician is diagnosing a truck with a complaint of slow air pressure build-up. The test shows it takes over three minutes for the pressure to rise from 85 to 100 PSI at governed RPM. Which of the following is the LEAST likely cause of this issue?
- A faulty low-pressure warning switch (Correct answer)
- A slipping compressor drive belt
- Excessive carbon buildup in the discharge line
- A leaking unloader valve on the compressor
Correct answer: A faulty low-pressure warning switch
The low-pressure warning switch is a sensor that activates a light or buzzer when system pressure drops below a safe level (typically 60 PSI); it has no role in the generation or build-up of air pressure. A slipping belt, a restricted discharge line, or a leaking unloader valve would all directly impede the compressor's ability to efficiently build pressure.
Question 2: During a routine service, a technician observes that the air governor cuts out at 145 PSI and the safety valve releases air. What is the MOST likely cause of this condition?
- A leaking air dryer purge valve
- A restricted compressor intake
- An improperly adjusted or faulty governor (Correct answer)
- A ruptured service brake diaphragm
Correct answer: An improperly adjusted or faulty governor
The air governor is responsible for controlling the compressor's loading and unloading cycles to maintain system pressure within a preset range (typically 120-135 PSI). If the governor is faulty or misadjusted and fails to signal the compressor to cut out, pressure will continue to build until the safety valve (set around 150 PSI) is forced open to prevent system damage. The other options would lead to low pressure, not excessive pressure.
Question 3: A driver reports that the service brakes on their truck are slow to release, causing a dragging sensation. Which of the following components is MOST likely to cause this specific symptom?
- A malfunctioning automatic slack adjuster
- A saturated air dryer desiccant cartridge
- A sticking quick-release or relay valve (Correct answer)
- A leaking spring brake chamber
Correct answer: A sticking quick-release or relay valve
Quick-release and relay valves are designed to speed up the release of air from the brake chambers when the brake pedal is released. If one of these valves is sticking or its exhaust port is restricted, it will trap air in the lines, preventing the brake chambers from retracting quickly and causing the brakes to drag. A faulty slack adjuster would affect application, a saturated dryer causes moisture issues, and a leaking spring brake chamber would cause an air leak, not slow release.
Question 4: When performing an applied air loss test, a technician chocks the wheels, releases the spring brakes, and applies firm pressure to the service brake pedal with the engine off. What is the maximum allowable pressure drop for a straight truck in one minute after the initial pressure drop?
- 1 PSI
- 2 PSI
- 3 PSI (Correct answer)
- 4 PSI
Correct answer: 3 PSI
According to standard air brake system test procedures, after the initial pressure drop from the brake application, the maximum allowable leakage rate for a straight truck or bus is 3 PSI in one minute. For a combination vehicle (tractor-trailer), the limit is 4 PSI per minute.
Question 5: A coach is brought in with the complaint that the spring parking brakes will not release. The air system builds to the correct cut-out pressure, and there are no audible leaks. Which of the following is a plausible cause for this failure?
- A failed service brake relay valve
- A malfunctioning inversion valve (Correct answer)
- A clogged air dryer filter
- Excessive slack adjuster free play
Correct answer: A malfunctioning inversion valve
The inversion valve controls the application and release of the spring brakes. It uses pilot air pressure to hold off the spring brakes. If the inversion valve malfunctions, it can prevent air pressure from being supplied to the spring brake chambers to compress the powerful springs, thus keeping the parking brakes applied even when the dash valve is pushed in. The other components listed are related to the service brakes or air supply, not specifically the release of the spring brakes.
Question 6: A technician is diagnosing a vehicle that pulls to the left only when the service brakes are applied. The foundation brake components and tire pressures are confirmed to be good. Which of the following is the MOST likely cause?
- A seized S-cam roller on the right side
- A restricted air line to the right side brake chambers (Correct answer)
- A faulty automatic slack adjuster on the left side
- A leaking diaphragm in the left side brake chamber
Correct answer: A restricted air line to the right side brake chambers
A restricted air line to the right side brake chambers would delay or reduce the application force on that side. This imbalance would cause the left side brakes, which are receiving full and immediate pressure, to apply more forcefully, resulting in the vehicle pulling to the left. A seized roller on the right would likely cause dragging or noise but not necessarily a pull under braking. A faulty slack adjuster or leaking diaphragm on the left side would reduce braking force on the left, causing a pull to the right.
310T Air Brake Systems Diagnosis Questions and Answers 1
A technician is diagnosing a truck with a complaint of slow air pressure build-up.
The test shows it takes over three minutes for the pressure to rise from 85 to 100 PSI at governed RPM.
Which of the following is the LEAST likely cause of this issue?