CDL Test: Air brakes questions and answers

1. AService, parking and emergency brakes
2. BFront, rear and trailer brakes
3. CEngine, exhaust and hydraulic brakes
4. DDisc, drum and band brakes

Answer: A. Service brakes for normal stops, parking brakes, and emergency brakes when the service system fails.

1. A60 psi
2. B125 psi
3. C200 psi
4. D90 psi

Answer: B. Cut-out is about 125 psi; cut-in is about 100 psi.

1. AOnce a month
2. BOnly when water is visible
3. CAt the end of each day of driving
4. DNever

Answer: C. Water and oil collect in the tanks and can freeze or damage the brakes.

1. A60 psi
2. B100 psi
3. C20 psi
4. D150 psi

Answer: D. If the safety valve releases air, something is wrong — have a mechanic fix it.

1. A60 psi
2. B100 psi
3. C20 psi
4. D125 psi

Answer: A. When the warning comes on, stop safely as soon as possible.

1. A100 to 125 psi
2. B20 to 45 psi
3. C60 to 80 psi
4. D0 psi only

Answer: B. Spring brakes apply at 20-45 psi (typically 20-30); stop before that happens.

1. ADisc brake
2. BBand brake
3. CS-cam drum brake
4. DWedge brake

Answer: C. S-cam drum brakes are the most common foundation brakes.

1. A1/8 inch
2. B3 inches
3. C6 inches
4. D1 inch

Answer: D. More than about one inch of movement means the brake probably needs adjustment.

1. A2 psi per minute
2. B10 psi per minute
3. C5 psi per minute
4. D1 psi per hour

Answer: A. Single vehicle: under 2 psi/min; combination: under 3 psi/min.

1. A1 psi per minute
2. B4 psi per minute
3. C10 psi per minute
4. D20 psi per minute

Answer: B. Applied-brake leakage: no more than 3 psi/min (single) or 4 psi/min (combination).

1. A10 minutes
2. B3 minutes
3. C45 seconds
4. D5 seconds

Answer: C. Dual systems: 85 to 100 psi within 45 seconds (at the manufacturer's specified RPM).

1. A20 psi
2. B45 psi
3. C60 psi
4. D100 psi in both systems

Answer: D. Wait for at least 100 psi in both the primary and secondary systems.

1. AAt least half a second
2. BAbout 5 seconds
3. CNo delay at all
4. DAbout 10 seconds

Answer: A. Air takes time to flow; at 55 mph brake lag adds about 32 feet, for a total stopping distance over 450 feet.

1. AHold the brakes lightly all the way down
2. BBrake firmly until you reach about 35 mph, release, and repeat when you reach 40
3. CUse only the parking brake
4. DShift to neutral and coast

Answer: B. Use engine braking in a low gear, then snub-brake to about 5 mph below safe speed for about 3 seconds at a time.

1. AThe vehicle is empty
2. BIt is daytime
3. CThe brakes are very hot or very wet in freezing temperatures
4. DYou are on level ground

Answer: C. Hot brakes can be damaged and wet brakes can freeze; use wheel chocks instead.

1. AKeep driving to the next terminal
2. BPump the brakes to build pressure
3. CSpeed up to reach help
4. DStop and safely park as soon as possible

Answer: D. Controlled braking is possible only while enough air remains in the tanks.

1. ALet air out faster than the compressor can replace it
2. BRecharge the air tanks
3. CCool the brakes
4. DAdjust the slack adjusters

Answer: A. Fanning the brakes wastes air.

1. AYour tires are low
2. BThe brakes are fading
3. CThe compressor is overcharging
4. DEverything is normal

Answer: B. Needing more application pressure for the same result is a sign of brake fade.

1. ACheck the oil level before driving
2. BDrain the oil at the end of each day
3. CAdd alcohol to the oil in cold weather
4. DReplace the oil every time the governor cuts out

Answer: A. A compressor with its own oil supply needs its oil level checked before you drive.

1. AThe safety relief valve
2. BThe governor
3. CThe stop light switch
4. DThe treadle valve

Answer: B. The governor stops and starts the compressor at the cut-out and cut-in pressures.

1. A60 psi
2. B85 psi
3. C100 psi
4. D125 psi

Answer: C. Pumping should start at about 100 psi and stop at about 125 psi.

1. AEnough air for only one brake application
2. BEnough air to drive about an hour without braking limits
3. CNo braking at all, because the spring brakes come on at once
4. DEnough air to use the brakes several times

Answer: D. The tanks hold enough air for several brake applications even if the compressor fails.

1. AIt can freeze in cold weather and cause brake failure
2. BIt makes the brake shoes wear evenly
3. CIt raises the governor cut-out pressure
4. DIt lowers the safety valve setting

Answer: A. Water collects in the tanks and can freeze, causing brake failure.

1. ABy pressing the brake pedal twice
2. BBy turning a quarter turn or pulling a cable
3. CBy pulling out the yellow parking brake knob
4. DBy switching on an electric heater

Answer: B. Manual drain valves open with a quarter turn or by pulling a cable.

1. AOverheating of the brake drums on long downgrades
2. BOverpressure in the first tank when the governor fails
3. CFreezing of the automatic drain in cold weather
4. DSlow brake response after the engine is shut off

Answer: C. Electric heaters keep the automatic drain from freezing in cold weather.

1. ATo cool the brake drums and reduce fade on long downgrades in hot weather
2. BTo lubricate the compressor pistons and keep them from sticking when the engine is cold
3. CTo raise the pressure in the air tanks so the brakes apply faster in cold weather
4. DTo reduce the risk of ice in the valves and other parts in cold weather

Answer: D. The alcohol evaporator helps keep ice from forming in airbrake valves and parts.

1. AEvery day
2. BOnce a week
3. COnce a month
4. DOnly when the low air warning comes on

Answer: A. Check the alcohol container and fill it up as necessary every day in cold weather.

1. AOn the foot valve connected to the brake pedal
2. BIn the first tank the air compressor pumps air to
3. CIn every brake chamber next to the slack adjuster
4. DOn the trailer air line at the gladhand coupler

Answer: B. The safety relief valve is in the first tank the compressor fills and protects the system from too much pressure.

1. AThis is normal when the brakes are applied
2. BThe governor is working correctly
3. CSomething is wrong and a mechanic should fix it
4. DThe spring brakes are about to release

Answer: C. A safety valve that releases air means a fault exists that a mechanic should fix.

1. AAir flows back into the tanks from the brake chambers and pressure rises
2. BThe pressure stays the same because the air is reused in a sealed system
3. CThe governor shuts off the compressor until the next time you start the engine
4. DSome compressed air leaves the system and the compressor must make it up

Answer: D. Releasing the brakes lets air out of the system, and the compressor has to replace it.

1. AThe cam is shaped like the letter S
2. BIt is made by a company starting with S
3. CIt is used only on single axles
4. DIt uses a spring to apply the brakes

Answer: A. The s-cam is so called because it is shaped like the letter S.

1. AA wedge
2. BA power screw
3. CA brake drum
4. DA spring-loaded shoe

Answer: B. Air disc brakes use a power screw instead of an s-cam to clamp the rotor.

1. AAn S-shaped cam between the ends of two brake shoes
2. BA caliper directly against the face of a brake rotor
3. CA wedge directly between the ends of two brake shoes
4. DA spring directly against the inside of the brake drum

Answer: C. In wedge brakes the push rod forces a wedge between the shoe ends, spreading them against the drum.

1. AOne gauge with one needle that shows the combined pressure of both halves
2. BA gauge for the trailer supply line and one for the brake chambers
3. CNo pressure gauges, only a low air warning light and a buzzer
4. DA gauge for each half of the system, or one gauge with two needles

Answer: D. Dual systems have a gauge for each half, or a single gauge with two needles.

1. ABrakes out of adjustment, air leaks, or mechanical problems
2. BProperly adjusted brakes, a working governor, and tight air lines
3. COverinflated tires or a lightly loaded trailer
4. DCold weather and a fully charged air system, with no defects

Answer: A. Needing more pressure can be caused by fade, brakes out of adjustment, air leaks, or mechanical problems.

1. ASounds a loud buzzer into the cab when pressure rises above 60 psi
2. BDrops a mechanical arm into your view when pressure falls below 60 psi
3. CTurns on a red warning light on the dash when the brakes are applied
4. DShuts off the compressor and applies the trailer brakes automatically

Answer: B. The wig wag drops a mechanical arm into view when system pressure drops below 60 psi.

1. AIt rises out of view by itself above 60 psi
2. BIt resets when the engine is turned off
3. CIt must be placed in the "out of view" position manually
4. DIt stays in view until the brake pedal is pressed

Answer: C. The manual reset type has to be moved to the out-of-view position by hand, and will not stay until pressure is above 60 psi.

1. A20-30 psi
2. B45-50 psi
3. C100-125 psi
4. D80-85 psi

Answer: D. On large buses it is common for the low pressure warning to signal at 80-85 psi.

1. AAn electric switch that works by air pressure turns on the brake lights
2. BA mechanical arm swings out from the rear of the trailer to signal drivers behind
3. CThe low air pressure buzzer sounds outside the cab so drivers behind can hear
4. DThe parking brake knob pops out and turns on the rear marker lamps

Answer: A. An air-pressure-operated electric switch turns on the brake lights when you apply the airbrakes.

1. ADoubles the air pressure to the front brakes
2. BCuts the normal air pressure to the front brakes by half
3. CReleases the front brakes completely
4. DApplies only the rear brakes at 60 psi

Answer: B. In the slippery position the valve cuts the normal front brake air pressure by half.

1. AThe brakes are put on very lightly (under 20 psi application pressure)
2. BThe parking brake is applied and the spring brakes are holding the vehicle
3. CThe brakes are put on very hard (60 psi or more application pressure)
4. DThe vehicle is stopped on a wet or icy road with the engine running

Answer: C. Automatic valves cannot be controlled by the driver and let full air through only on very hard applications, 60 psi or more.

1. ABecause hydraulic fluid can freeze
2. BBecause the compressor cannot keep up with the governor
3. CBecause the brake lining wears out too fast
4. DBecause air pressure can eventually leak away

Answer: D. Spring brakes use mechanical force because air pressure can eventually leak away.

1. AThe brakes being in adjustment
2. BThe size of the air compressor
3. CThe color of the parking brake knob
4. DThe temperature of the air tanks

Answer: A. If brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.

1. ARound, blue, twist-to-lock control knob
2. BDiamond-shaped, yellow, push-pull knob
3. CSquare, red, spring-loaded push button
4. DOctagonal, green, lever-type handle

Answer: B. Newer vehicles use a diamond-shaped, yellow, push-pull control knob for the parking brakes.

1. AThe compressor would stop pumping and the tanks would lose all their air
2. BThe automatic tank drains would open and let the stored air escape
3. CThe brakes could be damaged by the combined force of springs and air pressure
4. DThe low air warning would shut off and the pressure gauge would stick

Answer: C. Combined spring and air force can damage the brakes.

1. AA limiting valve
2. BA tractor protection valve
3. CA safety relief valve
4. DA modulating valve

Answer: D. A modulating valve lets you apply the spring brakes gradually.

1. AMove the lever as far as it will go and hold it with the locking device
2. BMove the lever halfway and release it
3. CLeave the lever in the released position and chock the wheels
4. DPush the lever in until the buzzer stops

Answer: A. The lever must be fully applied and held by the locking device.

1. ATo fill the trailer air lines when the tractor compressor has failed
2. BTo release the spring brakes so you can move the vehicle in an emergency
3. CTo power the low air pressure warning when the main tanks are empty
4. DTo supply air for cooling the brake drums on long downgrades in hot weather

Answer: B. The separate tank can release the spring brakes so the vehicle can be moved in an emergency.

1. AUnlimited times, as it is refilled by the compressor
2. BExactly once
3. COnly a few times, so plan carefully when moving
4. DAs many times as the brake pedal is pressed

Answer: C. There is only enough air in the separate tank for a few releases; otherwise you may be stopped in a dangerous location.

1. AMarch 1, 1991
2. BMarch 1, 1975
3. CMarch 1, 2005
4. DMarch 1, 1997

Answer: D. Tractors built on or after March 1, 1997 and other air brake vehicles built on or after March 1, 1998 must have ABS.

1. ACheck the certification label for the date of manufacture
2. BLook for a red ABS lamp mounted on the trailer's rear bumper
3. CCount the air tanks and compare with the number of axles
4. DTest the safety valve and note the pressure at which it opens

Answer: A. The certification label shows the date of manufacture, which tells you if the vehicle has ABS.

1. AOn the right side near the landing gear
2. BOn the left side, either on the front or rear corner
3. COn the instrument panel of the tractor only
4. DUnder the trailer by the air tanks

Answer: B. Trailers have the yellow ABS lamp on the left side, front or rear corner.

1. AStay on until you reach five mph
2. BStay off completely
3. CCome on for a bulb check, then go out quickly
4. DFlash until the brakes are applied

Answer: C. The lamp should light briefly for a bulb check and then go out.

1. AToo much air pressure in the tanks
2. BA frozen automatic drain
3. CBrake lag at 55 mph
4. DLost ABS control at one or more wheels

Answer: D. A lit lamp means ABS control may be lost at one or more wheels.

1. AAn electronic control unit (ECU) and wheel speed sensor wires from the back of the brakes
2. BA diamond-shaped yellow knob and a separate air tank mounted beside the landing gear
3. CA wig wag arm on the left side and a mechanical slack adjuster at each wheel
4. DAn alcohol evaporator and an electric heating device on the automatic tank drain

Answer: A. Look under the vehicle for the ECU and wheel speed sensor wires.

1. AIt doubles it
2. BIt neither decreases nor increases it
3. CIt decreases it on dry roads
4. DIt replaces the regular brakes

Answer: B. ABS is an addition to the normal brakes and does not change normal braking capability.

1. AIt always shortens your stopping distance on every road surface
2. BIt removes brake lag so the brakes apply instantly at any speed
3. CIt helps you keep the vehicle under control during hard braking
4. DIt prevents brake fade on long downgrades by cooling the drums

Answer: C. ABS does not necessarily shorten stopping distance, but helps keep control in hard braking.

1. AOne air system with two compressors that share a single set of tanks
2. BTwo sets of brake pedals, one for the tractor and one for the trailer
3. CA hydraulic backup system that takes over when the air pressure is lost
4. DTwo separate airbrake systems that use a single set of brake controls

Answer: D. A dual system has two separate systems, each with its own tanks, hoses and lines, using one set of controls.

1. AThe front axle (and possibly one rear axle)
2. BThe rear axle or axles (and possibly the trailer)
3. CThe trailer axles (and possibly one tractor axle)
4. DThe tandem axles (and possibly a dolly axle)

Answer: A. The primary system usually serves the rear axle(s); the other serves the front axle and possibly one rear axle.

1. A20 psi
2. B60 psi
3. C45 psi
4. D125 psi

Answer: B. The shut-off value is set by the manufacturer and must be greater than 60 psi.

1. AThe vehicle will stop faster because the other system carries all the braking
2. BThe trailer brakes lock up and the tractor brakes release automatically
3. CThe front or rear brakes will not operate fully and it will take longer to stop
4. DThe compressor shuts off permanently and cannot rebuild the air pressure

Answer: C. Either the front or rear brakes will not work fully, so stopping takes longer.

1. APark on a slope with the parking brakes on
2. BRun the engine at a fast idle with the brakes applied
3. CDrain the air tanks completely
4. DPark on level ground, chock the wheels, and turn off the parking brakes

Answer: D. Chock the wheels and release the parking brakes so the slack adjusters can be moved.

1. AOut-of-adjustment brakes
2. BCracked windshields
3. CWorn tires
4. DMissing mud flaps

Answer: A. Out-of-adjustment brakes are the most common problem found in roadside inspections.

1. A1975
2. B1991
3. C1997
4. D1998

Answer: B. All vehicles built since 1991 have automatic slack adjusters, but they must still be checked.

1. AAdjust them by hand yourself at the start of every trip
2. BIgnore it because automatic adjusters fix themselves over time
3. CTake the vehicle to a repair facility as soon as possible
4. DDrive only on dry roads until the next scheduled inspection

Answer: C. Manual adjustment only masks a mechanical problem; get the problem corrected at a repair facility.

1. AOne inch
2. BThe width of the friction area
3. COne quarter of the drum diameter
4. DOne half the width of the friction area

Answer: D. Cracks longer than one half the width of the friction area are not allowed.

1. ATurn the electrical power on and step on and off the brake pedal to reduce tank pressure
2. BLeave the electrical power off and pull out the parking brake knob to drain the tank pressure
3. CRun the engine at fast idle and watch the gauge until the buzzer sounds at 120 psi
4. DOpen the tank drain valves fully and check that the buzzer sounds when the tanks are empty

Answer: A. Reduce pressure by fanning the brake pedal; the signal must come on before pressure drops below 60 psi.

1. APull out the parking brake knob with full pressure
2. BContinue to fan off the air pressure by stepping on and off the brake pedal
3. COpen the tank drain valves
4. DPush the brake pedal down while the spring brakes are on

Answer: B. The parking brake valve should pop out when pressure falls to the manufacturer's specification (20 to 45 psi).

1. A85 to 100 psi within 45 seconds at 1,500 rpm
2. B20 to 45 psi within 1 minute at 600-900 rpm idle
3. C50 to 90 psi within 3 minutes at 600-900 rpm idle
4. D100 to 125 psi within 5 minutes at 1,200-1,500 rpm

Answer: C. Single systems typically build 50 to 90 psi within 3 minutes with the engine idling at 600-900 rpm.

1. ATwo psi in one minute
2. BFour psi in one minute
3. CTen psi in one minute
4. DThree psi in one minute

Answer: D. The limit is two psi per minute for single vehicles and three psi per minute for combinations.

1. AThree psi in one minute
2. BTwo psi in one minute
3. CFour psi in one minute
4. DFive psi in one minute

Answer: A. For single vehicles more than three psi per minute is too much (four for combinations).

1. AApply it and drive at five mph
2. BPut it on and gently pull against it in a low gear
3. CRelease it and press the brake pedal
4. DApply it and drain the tanks

Answer: B. Stop, set the parking brake and gently pull against it in low gear to see that it holds.

1. AFifteen mph
2. BTwenty-five mph
3. CFive mph
4. DThirty-five mph

Answer: C. Test at about five mph and note any pulling to one side, unusual feel, or delayed stopping.

1. AThe vehicle jackknifes
2. BThe vehicle spins
3. CThe brakes release automatically
4. DYou lose steering control

Answer: D. When steering wheels lock up you lose steering control; when other wheels lock you may skid, jackknife or spin.

1. AIf you always drive a straight truck or combination with working ABS on all axles
2. BIf you drive a combination where only the tractor has working ABS
3. CIf you drive a combination where only the trailer has working ABS
4. DIf the road is dry and the vehicle is loaded below its legal weight

Answer: A. This is the only exception to braking only as hard as needed to stay in control.

1. AUp to five seconds
2. BUp to one second
3. CUp to ten seconds
4. DLess than a tenth of a second

Answer: B. If you re-apply before the wheels start rolling, the vehicle won't straighten out.

1. A10 feet
2. B75 feet
3. C32 feet
4. D132 feet

Answer: C. With airbrakes, lag distance at 55 mph adds about 32 feet to total stopping distance.

1. AAbout 200 feet
2. BAbout 300 feet
3. COver 750 feet
4. DOver 450 feet

Answer: D. Perception + reaction + brake lag + braking distance totals over 450 feet at 55 mph.