Air Brake Systems Diagnosis Flashcards

An air compressor is pumping oil into the air system. Which component failure is the MOST common cause?

Answer: Worn compressor piston rings or cylinder walls

Worn piston rings or cylinder walls allow crankcase oil to bypass into the compression chamber and enter the air system.

A trailer's spring brakes release when connected to the tractor, but then slowly re-apply while driving. What is the MOST likely cause?

Answer: Trailer relay emergency valve leaking supply air

A leaking trailer relay emergency valve gradually loses supply pressure from the spring brake chambers, allowing the springs to progressively re-apply.

When inspecting brake drums, a technician finds a blue-gray discoloration on the drum friction surface. This indicates:

Answer: Drum has been overheated and may be heat-cracked

Blue-gray discoloration is a classic sign of thermal overloading; the drum should be inspected for heat cracks and measured before returning to service.

A technician is using a brake force meter (decelerometer) to perform a road test. For a Class 8 vehicle, what is the minimum acceptable deceleration rate to pass?

Answer: 14 ft/s²

FMCSA requires a minimum deceleration of 14 ft/s² (approximately 0.43g) for combination vehicles during a brake performance road test.

What is the function of the inversion valve (also called a modulating valve) in a spring brake system?

Answer: It increases spring brake chamber pressure proportionally as service brake pressure decreases

An inversion valve applies increasing air pressure to the spring brake chamber in inverse proportion to a decreasing signal, allowing modulated spring brake application.

A vehicle's ABS wheel speed sensor produces an erratic signal. Which condition would MOST likely cause this?

Answer: Excessive air gap between the sensor and tone ring

An excessive air gap between the sensor and exciter ring weakens the magnetic signal, causing the ECU to receive erratic or missing speed data.

After replacing a brake chamber diaphragm, the technician must verify proper operation by:

Answer: Measuring push rod stroke at 90 psi application pressure and confirming it is within the maximum stroke limit

After diaphragm replacement, a functional check at 90 psi confirms the chamber delivers proper stroke and the new diaphragm is sealing correctly.