4.0 Air Brakes CDL-A Practice Quiz
60 exam-style questions covering 20% of the CDL-A exam. Instant feedback on every answer, progress tracking, no signup required.
This domain is part of the CDL Class A practice test. Each question is tagged by exam objective and difficulty so you can drill exactly the areas you need.
Sample Questions
Air brake systems use a specific component to generate the compressed air that operates the brakes. According to the CDL manual, what is the basic function of the air compressor in an air brake system?
- A. It pumps air into the air storage tanks (reservoirs)
- B. It releases compressed air through the foot valve to apply the brakes
- C. It converts hydraulic pressure into pneumatic pressure for the slack adjusters
- D. It powers the spring brakes by mechanical force when the engine is off
The CDL manual states that the air compressor pumps air into the air storage tanks (reservoirs) and is connected to the engine through gears or a v-belt.
The CDL manual gives several alternate names for the brake pedal in an air brake system. Which set of names matches the manual's terminology?
- A. Brake pedal, foot valve, and treadle valve
- B. Slack adjuster and brake chamber
- C. Glad hand and tractor protection valve
- D. S-cam and wedge driver
The CDL manual states the brake pedal is also called the foot valve or the treadle valve.
The air compressor governor controls when the compressor pumps air into the storage tanks. According to the CDL manual, around what cut-out and cut-in pressures does the governor typically operate?
- A. Cut-out at about 60 psi, cut-in at about 30 psi
- B. Cut-out at about 125 psi, cut-in at about 100 psi
- C. Cut-out at about 150 psi, cut-in at about 125 psi
- D. Cut-out at about 200 psi, cut-in at about 175 psi
The CDL manual states the governor stops the compressor at the cut-out level around 125 psi and allows pumping again at the cut-in level around 100 psi.
The most common foundation brake on heavy commercial vehicles uses a specific cam shape that gives the device its name. According to the CDL manual, what is the s-cam, and how does it work in the brake?
- A. A spring-loaded cam that pushes a wedge between the brake pads
- B. A cam shaped like the letter S that, when rotated by the slack adjuster, forces the brake shoes apart against the inside of the drum
- C. A solenoid-actuated cam that triggers ABS during hard stops
- D. A coiled cam used in air-over-hydraulic conversions for medium trucks
The CDL manual describes the s-cam as a cam shaped like the letter S that, when rotated through the brake camshaft by the slack adjuster, forces the brake shoes apart against the inside of the brake drum.
A visible low air pressure warning signal is required on every CMV with air brakes. According to the CDL manual, the warning must come on before air pressure in the tanks falls below what value?
- A. Below 30 psi
- B. Below 60 psi (or one half the compressor governor cutout pressure on older vehicles)
- C. Below 100 psi
- D. Below 125 psi
The CDL manual states the warning signal must come on before the air pressure in the tanks falls below 60 psi, or below one half the governor cutout pressure on older vehicles.
A safety relief valve is installed in the first air tank to protect the system from overpressure. According to the CDL manual, at approximately what pressure is the safety valve usually set to open?
- A. About 60 psi
- B. About 100 psi
- C. About 150 psi
- D. About 250 psi
The CDL manual states the safety valve is usually set to open at 150 psi to protect the tank and rest of the system from too much pressure.
The CDL manual identifies one specific type of foundation brake as the most common on heavy commercial vehicles. Which type does the manual call the most common?
- A. Wedge brake at every wheel
- B. Air-operated disc brake at every wheel
- C. S-cam drum brake at each wheel
- D. Hydraulic-over-air brake at each wheel
The CDL manual states foundation brakes are used at each wheel and that the most common type is the s-cam drum brake.
Compressed air contains some water and oil that collect at the bottom of the air tanks. According to the CDL manual, what is the proper drainage practice for tanks equipped only with manual drain valves?
- A. Drain only when the safety valve releases air
- B. Drain only when the air dryer indicator lamp illuminates
- C. Drain monthly during routine preventive maintenance only
- D. Drain the tanks completely at the end of each day of driving
The CDL manual instructs drivers to drain manually-valved tanks completely at the end of each day of driving, because water can freeze in cold weather and cause brake failure.
Key Terms in This Domain
- Spring brakes (parking): Powerful springs apply the brakes when air pressure is released; will activate automatically when system pressure drops below 20-45 psi
- Air brake test (in-cab portion): Static leakage, applied leakage, low-air warning, spring brake pop-out, parking brake test, service brake test
- Air-loss leakage rate (single): No more than 2 psi per minute with engine off, full pressure, brakes released
- Low-air warning test: With engine off and brakes fully applied to release pressure, the warning must activate before pressure drops below 60 psi
- Spring brake pop-out test: Continue releasing pressure; spring brakes should automatically apply between 20-45 psi
- Foot brake (treadle valve): Driver-operated valve that delivers air pressure to the service brakes
- Trailer hand valve: Optional separate control that applies only the trailer brakes; should NOT be used for parking and should not be used to prevent jackknifing
- Brake chamber: Converts air pressure to mechanical force to push the slack adjuster and apply the brakes
- Air-loss leakage rate (combination): No more than 3 psi per minute with engine off, full pressure, brakes released
- Applied leakage rate (combination): No more than 4 psi per minute with engine off, full reservoir pressure, brakes fully applied
Link to this quiz
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Other CDL-A Domains
- 1.0 Driver Qualifications and CDL Fundamentals
- 2.0 Driving Safely and On-Road Operation
- 3.0 Transporting Cargo Safely
- 5.0 Combination Vehicles
- 6.0 Pre-Trip Vehicle Inspection
- 7.0 Basic Vehicle Control and Maneuvering
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