310T Air Brakes and Braking Systems 3 — Questions and Answers
Question 1: What type of valve modulates air pressure to each wheel independently during ABS activation?
- Relay valve
- Modulator valve (ABS solenoid valve) (Correct answer)
- Quick-release valve
- Limiting valve
Correct answer: Modulator valve (ABS solenoid valve)
ABS modulator (solenoid) valves cycle between hold, dump, and reapply modes to control wheel slip at each monitored axle end.
Question 2: In a dual-circuit air brake system, which reservoir is pressurized first by the compressor?
- Front (primary) circuit reservoir
- Rear (secondary) circuit reservoir
- Wet (supply) tank (Correct answer)
- Trailer reservoir
Correct answer: Wet (supply) tank
Air from the compressor flows first into the wet (supply) tank where moisture collects, then through check valves into the primary and secondary circuit reservoirs.
Question 3: What is the minimum air pressure at which the low-pressure warning device must activate per FMCSA regulations?
- 45 psi
- 60 psi (Correct answer)
- 80 psi
- 90 psi
Correct answer: 60 psi
FMCSA (49 CFR 393.51) requires the low air pressure warning to activate before reservoir pressure falls below 60 psi.
Question 4: A relay valve is used in air brake systems primarily to:
- Reduce air pressure to the front axle brakes
- Shorten brake application and release time for remote chambers (Correct answer)
- Protect the tractor air system from trailer air loss
- Balance pressure between primary and secondary circuits
Correct answer: Shorten brake application and release time for remote chambers
Relay valves use a small control signal from the brake treadle to open a local large-volume air supply to remote brake chambers, reducing lag time.
Question 5: On a combination vehicle, which valve automatically closes to protect the tractor air supply if trailer air pressure drops rapidly?
- Glad hand check valve
- Tractor protection valve (Correct answer)
- Double-check valve
- Inversion valve
Correct answer: Tractor protection valve
The tractor protection valve senses a sudden drop in trailer supply line pressure and shuts off air flow to the trailer, protecting tractor system pressure.
Question 6: What is the function of the inversion (fail-safe) valve used with spring brake chambers?
- Inverts the pressure signal so spring brakes apply when service pressure is lost (Correct answer)
- Doubles the spring brake hold-off pressure for hill starts
- Converts service brake application into spring brake application
- Allows manual release of spring brakes without air
Correct answer: Inverts the pressure signal so spring brakes apply when service pressure is lost
An inversion valve routes service brake air to release spring brakes when the park circuit fails, using the inverse relationship between circuits to maintain brake control.
Question 7: Brake fade on a heavily loaded truck descending a long grade is MOST likely caused by:
- Overloaded air reservoirs losing pressure
- Heat buildup reducing friction between lining and drum (Correct answer)
- ABS modulator valve cycling too frequently
- Slack adjuster over-travel beyond rated stroke
Correct answer: Heat buildup reducing friction between lining and drum
Sustained braking generates heat that raises drum and lining temperatures, reducing the coefficient of friction and causing fade.
310T Air Brakes and Braking Systems 3
What type of valve modulates air pressure to each wheel independently during ABS activation?