310T Air Brake Systems Diagnosis 3 — Questions and Answers
Question 1: The secondary reservoir pressure is normal, but primary reservoir pressure will not build above 70 psi. The MOST likely cause is:
- A restricted primary reservoir check valve (Correct answer)
- The governor cutting out at 70 psi
- A faulty dual brake valve primary section leaking internally to atmosphere
- A clogged wet tank drain valve
Correct answer: A restricted primary reservoir check valve
A restricted check valve between the wet tank and primary reservoir limits air flow, preventing the primary circuit from reaching full system pressure.
Question 2: During a trailer service brake application, air exhausts from the trailer supply gladhand instead of the service gladhand. This indicates:
- Reversed gladhand connections at the trailer nose (Correct answer)
- A faulty trailer control valve
- A blown diaphragm in the trailer relay valve
- Low primary circuit pressure
Correct answer: Reversed gladhand connections at the trailer nose
Reversed gladhand connections cause service air to flow into the supply line, which vents through the emergency portion of the trailer control valve.
Question 3: A technician finds water and rust in the primary reservoir but not the secondary. What is the MOST likely cause?
- The secondary reservoir drain valve is stuck closed
- The primary reservoir check valve is leaking, bypassing the air dryer (Correct answer)
- Air dryer desiccant is saturated and not drying primary circuit air
- The governor is allowing the compressor to run too long
Correct answer: The primary reservoir check valve is leaking, bypassing the air dryer
A leaking primary reservoir check valve can allow unfiltered, unprocessed air to bypass the dryer and enter the primary reservoir directly.
Question 4: What is the purpose of the double-check valve in an air brake system?
- To prevent air from flowing backward from reservoirs to the compressor
- To supply the highest pressure source from either the primary or secondary circuit to a single output (Correct answer)
- To limit maximum brake application pressure to a safe level
- To equalize pressure between the primary and secondary reservoirs
Correct answer: To supply the highest pressure source from either the primary or secondary circuit to a single output
A double-check valve selects and passes the higher of two input pressures to a single output, commonly used to ensure either circuit can operate the spring brake control valve.
Question 5: A Class 8 truck repeatedly trips the low air pressure warning at 60 psi within minutes of starting the engine. What should be checked FIRST?
- Compressor belt tension and condition (Correct answer)
- All reservoir drain valves for leaks
- Brake chamber diaphragm integrity
- Governor cut-in pressure setting
Correct answer: Compressor belt tension and condition
A slipping or broken compressor drive belt prevents the compressor from building pressure, causing system pressure to remain low and triggering the warning buzzer.
Question 6: When diagnosing a foundation brake that runs hot on one axle end only, the technician should inspect the:
- Relay valve delivery pressure
- Automatic slack adjuster for proper operation and free movement (Correct answer)
- Air dryer purge valve timing
- Low-pressure warning switch calibration
Correct answer: Automatic slack adjuster for proper operation and free movement
A seized or improperly adjusted automatic slack adjuster keeps the brake partially applied, generating excessive heat at that wheel end.
Question 7: On a tractor-trailer combination, maximum pressure drop during a combined leakage test with brakes applied must not exceed:
- 3 psi per minute
- 4 psi per minute
- 6 psi per minute (Correct answer)
- 8 psi per minute
Correct answer: 6 psi per minute
FMCSA regulations allow up to 6 psi per minute pressure drop for combination vehicles during a static leakage test with service brakes applied.
310T Air Brake Systems Diagnosis 3
The secondary reservoir pressure is normal, but primary reservoir pressure will not build above 70 psi.
The MOST likely cause is: