Air Brakes and Braking Systems Flashcards

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

Answer: To allow the higher of two air pressure sources to pass through while blocking the lower

A double-check valve selects the higher pressure from two sources (e.g., service and emergency circuits) and routes it downstream, ensuring the highest available pressure is always used.

On a tractor-trailer combination, the trailer supply valve (also called the tractor protection valve control) is normally what color?

Answer: Red

The trailer supply valve knob is red (octagonal) and controls the emergency/supply air line to the trailer; yellow is the trailer park control.

What happens to the spring brakes when the parking brake control valve is pushed IN on a truck?

Answer: Spring brakes release

Pushing in the parking brake control valve (yellow knob) allows air into the spring brake chambers, compressing the springs and releasing the brakes.

Which component prevents moisture and contaminants from entering the air brake system from the compressor?

Answer: Air dryer

The air dryer removes moisture and oil vapor from compressor discharge air before it reaches the reservoirs, preventing corrosion and freezing.

What is the normal cut-in pressure for most air brake system compressors?

Answer: 80–85 psi

Most air brake governor settings have a cut-in (compressor starts pumping) around 80–85 psi and a cut-out around 120–130 psi.

A technician finds that a brake chamber's stroke is beyond the readjustment limit. What is the MOST likely cause?

Answer: Worn brake shoes/linings

Excessive pushrod stroke is most commonly caused by worn brake linings that increase the clearance between the lining and drum, requiring more chamber travel.

What does a quick-release valve do in an air brake system?

Answer: Exhausts air locally to speed up brake release

A quick-release valve exhausts air directly at the brake chamber location rather than routing it back through the treadle valve, speeding up brake release.