Air Brake Systems Diagnosis Flashcards

The secondary reservoir pressure is normal, but primary reservoir pressure will not build above 70 psi. The MOST likely cause is:

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.

During a trailer service brake application, air exhausts from the trailer supply gladhand instead of the service gladhand. This indicates:

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.

A technician finds water and rust in the primary reservoir but not the secondary. What is the MOST likely cause?

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.

What is the purpose of the double-check valve in an air brake system?

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.

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?

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.

When diagnosing a foundation brake that runs hot on one axle end only, the technician should inspect the:

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.

On a tractor-trailer combination, maximum pressure drop during a combined leakage test with brakes applied must not exceed:

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.