Air Brake Test Air Pressure Regulation & Control 2 — Questions and Answers
Question 1: What is the typical 'cut-out' pressure for an air brake compressor governor?
- 125 psi (Correct answer)
- 60 psi
- 90 psi
- 150 psi
Correct answer: 125 psi
The governor cuts out (stops the compressor from pumping) at approximately 125 psi, which is the normal maximum operating pressure for air brake systems.
Air brake systems operate within a pressure range controlled by the governor. When system pressure reaches the cut-out pressure (typically around 125 psi), the governor signals the compressor to stop pumping, preventing over-pressurization. When pressure drops to the cut-in pressure (around 100 psi), the governor restarts the compressor to rebuild pressure.
Question 2: What happens when air pressure drops below the governor 'cut-in' pressure?
- The compressor begins pumping air back into the system (Correct answer)
- The spring brakes automatically apply
- The low air pressure warning activates immediately
- The foot brake becomes inoperable
Correct answer: The compressor begins pumping air back into the system
When pressure falls to the cut-in level (approximately 100 psi), the governor signals the compressor to start pumping air to rebuild system pressure.
The governor monitors system air pressure and cycles the compressor on and off to maintain pressure within the normal operating range. The cut-in pressure is the lower threshold at which the compressor resumes pumping. This keeps system pressure between cut-in and cut-out levels, ensuring adequate air for braking at all times.
Question 3: What is the purpose of the safety relief valve in an air brake system?
- To release excess air if the governor fails and pressure becomes dangerously high (Correct answer)
- To drain moisture from the air tanks
- To maintain constant pressure in the brake lines
- To regulate air flow to individual wheel cylinders
Correct answer: To release excess air if the governor fails and pressure becomes dangerously high
The safety relief valve is a backup protection device that opens to vent air if the governor malfunctions and pressure exceeds a safe limit (usually set at 150 psi).
If the governor fails to cut out the compressor, pressure can build beyond safe levels and damage tanks, lines, and valves. The safety relief valve is preset (typically at 150 psi) to open automatically and release excess air before dangerous over-pressurization occurs. It should be tested periodically by pulling the test ring to confirm it opens and reseats properly.
Question 4: Which component in the air brake system separates oil and moisture from the compressed air before it enters the tanks?
- The air dryer (Correct answer)
- The governor
- The one-way check valve
- The quick release valve
Correct answer: The air dryer
The air dryer removes moisture and oil from the compressed air before it reaches the air tanks, preventing corrosion, freezing, and contamination of the brake system.
As the compressor compresses air, it also carries moisture vapor and compressor oil. An air dryer installed between the compressor and tanks captures this moisture and oil through desiccant or refrigerant-based methods. Proper air dryer function is critical in cold climates to prevent moisture from freezing in lines and valves, which could cause brake failure.
Question 5: What does it indicate if the air pressure does not rise from 50 to 90 psi within 3 minutes at engine idle?
- There is a problem with the air compressor or a significant air leak in the system (Correct answer)
- The governor needs adjustment
- The air tanks are full and do not need to charge
- Normal operation for cold weather conditions
Correct answer: There is a problem with the air compressor or a significant air leak in the system
FMCSA requires pressure to build from 50 to 90 psi within 3 minutes at idle, indicating the compressor is working properly; failure to do so signals a compressor or major leak problem.
During pre-trip inspection, drivers check that air pressure builds to operating range within the specified time. If pressure rises too slowly, it may indicate a failing compressor, worn compressor unloader valves, or a significant air leak. This is a critical check because adequate air pressure is essential for both service brakes and to hold off the spring brakes.
Question 6: What is the function of the one-way check valve located between the wet and dry tanks?
- It prevents air from flowing back from the dry tank to the wet tank if pressure drops (Correct answer)
- It drains oil from the wet tank automatically
- It balances pressure between the primary and secondary circuits
- It opens to release air when the driver applies the parking brake
Correct answer: It prevents air from flowing back from the dry tank to the wet tank if pressure drops
The one-way check valve ensures that air flows only from the wet tank to the dry/supply tanks and not backward, maintaining pressure in the system even if the wet tank develops a leak.
Air flows from the compressor into the wet (supply) tank, then through a one-way check valve into the primary and secondary tanks. If the wet tank or compressor develops a leak, the check valve prevents the dry tanks from losing their stored air, preserving brake function. This protects the driver's ability to stop the vehicle even with a supply-side failure.
Air Brake Test Air Pressure Regulation & Control 2
What is the typical 'cut-out' pressure for an air brake compressor governor?