310T Air Brake Systems Diagnosis 2 — Questions and Answers
Question 1: A truck's air dryer purge valve is constantly exhausting air. What is the MOST likely cause?
- Faulty compressor unloader valve
- Stuck open purge valve solenoid
- Excessive compressor oil carry-over clogging the desiccant (Correct answer)
- Governor set too low
Correct answer: Excessive compressor oil carry-over clogging the desiccant
Excessive oil carry-over from a worn compressor saturates the desiccant cartridge, reducing its efficiency and causing the purge cycle to vent continuously.
Question 2: During a brake application, the left front wheel locks up while the right front remains normal. This symptom MOST likely indicates:
- Low reservoir pressure on the primary circuit
- A seized left front brake chamber push rod
- A leaking right front service brake chamber diaphragm
- A stuck open ABS modulator valve on the left front (Correct answer)
Correct answer: A stuck open ABS modulator valve on the left front
A stuck open ABS modulator inlet valve prevents pressure release to the affected wheel, causing it to lock while the ABS system fails to cycle.
Question 3: When performing a static leakage test on a straight truck, the maximum allowable pressure drop per minute for the service brake system is:
- 2 psi
- 3 psi (Correct answer)
- 4 psi
- 6 psi
Correct answer: 3 psi
FMCSA regulations allow a maximum of 3 psi drop per minute for single-vehicle service brake systems during a static leakage test.
Question 4: A driver reports that the spring brakes apply automatically when going downhill on a long grade. The MOST likely cause is:
- Low spring brake modulator pressure
- Air compressor overheating and shutting off
- System air pressure dropping below the low-pressure warning threshold (Correct answer)
- Trailer supply valve leaking externally
Correct answer: System air pressure dropping below the low-pressure warning threshold
If sustained brake application drains the reservoir pressure below approximately 60 psi, the spring brakes automatically apply to prevent brake fade.
Question 5: What tool is used to accurately measure the push rod stroke of a brake chamber to determine if it exceeds the adjustment limit?
- Vernier caliper
- Dial indicator mounted to the chamber body
- Steel ruler applied at full brake application (Correct answer)
- Brake angle gauge
Correct answer: Steel ruler applied at full brake application
A steel ruler placed against the chamber body with the push rod at full application gives the stroke reading that is compared to the maximum stroke limit.
Question 6: A quick-release valve fails to exhaust air from the brake chambers quickly. Which symptom will the driver notice?
- Brake drag due to slow pressure release (Correct answer)
- High pedal effort during initial application
- Premature spring brake application
- Air dryer cycling too frequently
Correct answer: Brake drag due to slow pressure release
A restricted quick-release valve slows pressure exhaust from the chambers, delaying brake release and causing brake drag after the pedal is released.
Question 7: An S-cam brake assembly shows uneven lining contact, with the leading edge of the lining worn more than the trailing edge. This MOST likely indicates:
- Incorrect lining type installed
- Weak or broken return springs allowing the cam to over-rotate (Correct answer)
- Cam bushings worn allowing the cam to shift axially
- A cracked brake drum
Correct answer: Weak or broken return springs allowing the cam to over-rotate
Weak return springs allow the S-cam to rotate beyond its normal position, causing excessive contact at the leading edge and uneven lining wear.
310T Air Brake Systems Diagnosis 2
A truck's air dryer purge valve is constantly exhausting air.
What is the MOST likely cause?