310T Air Brakes and Braking Systems 2 — Questions and Answers
Question 1: What is the purpose of the double-check valve in an air brake system?
- To regulate air pressure to the rear brakes only
- To allow the higher of two air pressure sources to pass through while blocking the lower (Correct answer)
- To prevent air from flowing back into the supply reservoir
- To limit pressure to the spring brake chambers
Correct 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.
Question 2: On a tractor-trailer combination, the trailer supply valve (also called the tractor protection valve control) is normally what color?
- Blue
- Yellow
- Red (Correct answer)
- Green
Correct 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.
Question 3: What happens to the spring brakes when the parking brake control valve is pushed IN on a truck?
- Spring brakes apply
- Spring brakes release (Correct answer)
- Service brakes apply
- Nothing changes
Correct 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.
Question 4: Which component prevents moisture and contaminants from entering the air brake system from the compressor?
- Unloader valve
- Air dryer (Correct answer)
- Wet tank drain
- Pressure protection valve
Correct answer: Air dryer
The air dryer removes moisture and oil vapor from compressor discharge air before it reaches the reservoirs, preventing corrosion and freezing.
Question 5: What is the normal cut-in pressure for most air brake system compressors?
- 60–70 psi
- 80–85 psi (Correct answer)
- 100–120 psi
- 125–130 psi
Correct 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.
Question 6: A technician finds that a brake chamber's stroke is beyond the readjustment limit. What is the MOST likely cause?
- Worn brake shoes/linings (Correct answer)
- Low air pressure in the system
- Defective relay valve
- Cracked brake drum
Correct 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.
Question 7: What does a quick-release valve do in an air brake system?
- Quickly charges the reservoirs
- Exhausts air locally to speed up brake release (Correct answer)
- Prevents air loss if a brake line ruptures
- Reduces pressure spikes during hard braking
Correct 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.
310T Air Brakes and Braking Systems 2
What is the purpose of the double-check valve in an air brake system?