Air Brakes Practice Questions

Explanation: A low-pressure warning signal (light, buzzer or wig-wag) must activate before air pressure drops below 60 psi in the model CDL manual. A few state manuals, such as California's, use 55 psi — use the figure in your state manual.

Explanation: Spring brakes usually apply automatically somewhere between 20 and 45 psi. Stop safely before this happens.

Explanation: Brakes released: under 2 psi per minute for a single vehicle, under 3 psi for a combination. Brakes applied (after the initial drop): under 3 psi single, under 4 psi combination.

Explanation: For a combination vehicle with the brakes fully applied, leakage should be under 4 psi per minute (under 3 psi for a single vehicle).

Explanation: If a slack adjuster moves more than about 1 inch where the push rod attaches, it probably needs adjustment.

Explanation: Compressed air carries water and oil into the tanks. Drain manually operated tanks at the end of each day so the sludge cannot freeze or ruin brake parts.

Explanation: Stab braking (for vehicles without antilock brakes): brake fully, release when the wheels lock up, reapply once they roll again. Releasing too briefly can keep the vehicle sliding.

Explanation: Air takes a moment to flow to the brakes. At 55 mph this brake lag adds about 32 feet to the total stopping distance.

Explanation: Hot brakes can be damaged by the parking brake, and wet brakes can freeze to the drums in cold weather. Use wheel chocks instead until the brakes cool or dry.

Explanation: Dual systems split the brakes so a failure in one still leaves some braking. Typically the primary system works the regular brakes on the rear axle(s) and the secondary works the front axle.

Explanation: Brake fade happens when brakes get too hot. Prevent it by using a low gear and the brakes only in firm applications.

Explanation: Federal rules require ABS on truck tractors with air brakes built on or after March 1, 1997, and on air-braked trailers built on or after March 1, 1998.

Explanation: Low air warns that the brakes may soon stop working normally and the spring brakes may lock. Stop and safely park while you still have control, then find the leak.

Explanation: The alcohol evaporator puts alcohol into the air system to help stop water freezing in valves and lines in cold weather. Tanks still need regular draining.

Explanation: The supply pressure gauge shows tank pressure. An application pressure gauge, if fitted, shows how much pressure you are sending to the brakes.

Explanation: Air brakes combine the service brake (normal stopping), the parking brake, and the emergency brake system, which uses parts of the other two when the service system fails.

Explanation: Pressing the brake pedal (foot valve) sends air to the brake chambers. Releasing it lets that air out.

Explanation: Air pushes the push rod, which turns the slack adjuster and brake camshaft. The S-cam twists and forces the shoes apart against the inside of the drum.

Explanation: Powerful springs apply spring brakes; air pressure holds them back. If air pressure is lost, the springs put the brakes on.

Explanation: Each brake application uses air. Fanning (rapidly applying and releasing) can drop air pressure below a safe level.

Explanation: ABS is an addition to normal brakes. If it fails, the regular brakes still work normally, but the wheels can lock in hard stops. Get it serviced soon.

Explanation: The air compressor is driven by the engine through gears or a V-belt, and may be air-cooled or cooled by the engine cooling system.

Explanation: A one-way check valve lets air into the tank but stops it from leaking back out if the compressor or line fails.

Explanation: Controlled braking: brake as hard as possible without locking the wheels. If you need to steer or the wheels lock, release and reapply when you can.

Explanation: Perception (about 142 ft) + reaction (61 ft) + brake lag (32 ft) + braking (about 216 ft) gives roughly 450 feet — longer than a football field.

Explanation: Set the parking brake and gently try to pull forward in a low gear. The vehicle should not move.

Explanation: Automatic drains remove most water and oil, but check that they are working. Manual valves must be drained by hand, usually at the end of each day.

Explanation: If braking power is poor, check the air pressure and brake adjustment. Brakes out of adjustment are a common cause of weak braking and brake fade.

Explanation: The two systems are independent, so one can keep providing some braking if the other fails. Stop safely and have it repaired.

Explanation: Let the pressure build to normal — up to the governor cut-out, about 125 psi — and make sure the low-pressure warning has gone off before moving.

Explanation: The low air warning must come on before pressure falls below 60 psi (55 psi in a few state manuals, such as California's). Keep fanning: the spring brakes should apply between 20 and 45 psi.

Explanation: The compressor may be air cooled or cooled by the engine cooling system.

Explanation: A compressor may have its own oil supply or use engine oil. If it has its own supply, check the oil level before driving.

Explanation: Manual drains are opened by turning a quarter turn or pulling a cable. You must drain these tanks yourself at the end of each day of driving.

Explanation: Water and compressor oil collect in the bottom of the air tank, which is why each tank has a drain valve at the bottom.

Explanation: Check the alcohol container and fill it as needed every day during cold weather.

Explanation: An alcohol evaporator reduces ice risk, but daily tank drainage is still needed to remove water and oil unless the system has automatic drains.

Explanation: The safety valve sits in the first tank the compressor pumps air to and protects the tank and system from too much pressure.

Explanation: The brake pedal (foot or treadle valve) controls air pressure: pressing harder applies more pressure, letting up reduces it.

Explanation: Foundation brakes are used at each wheel, and the S-cam drum brake is the most common type. Wedge and disc brakes are less common.

Explanation: The wedge is pushed between the ends of the two shoes, forcing them apart and against the inside of the drum.

Explanation: Air acts on a brake chamber and slack adjuster, which turns a power screw instead of an S-cam.

Explanation: A dual system has a gauge for each half of the system, or a single gauge with two needles.

Explanation: The application gauge shows the pressure being applied to the brakes. Not all vehicles have one.

Explanation: Rising application pressure for the same speed means brake fade. Slow down and use a lower gear.

Explanation: A wig wag drops a mechanical arm into your view when system pressure falls too low.

Explanation: Large buses commonly have low-pressure warnings set to signal at 80 to 85 psi.

Explanation: The stop light switch is an electric switch worked by air pressure that turns on the brake lights when you apply the air brakes.

Explanation: In the slippery position, the limiting valve cuts the normal air pressure to the front brakes by half, which reduces stopping power.

Explanation: Front wheel braking is good under all conditions, and front wheel skids from braking are not likely even on ice. Keep the control in normal.

Explanation: Automatic limiting valves reduce front brake air except during very hard applications of 60 psi or more. The driver cannot control them.

Explanation: Air can leak away over time, so parking and emergency brakes must be held on mechanically, usually by spring brakes.

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

Explanation: Newer vehicles use a yellow, diamond-shaped push-pull knob to control the parking (spring) brakes.

Explanation: Pressing the pedal with the spring brakes on can damage the brakes through the combined force of springs and air. Make it a habit not to do it.

Explanation: A modulating valve is a spring-loaded dash control that applies the spring brakes gradually, so you can control them if the service brakes fail.

Explanation: To park, move the modulating lever as far as it goes and lock it in place.

Explanation: The separate tank holds only enough air for a few releases, so plan carefully or you may be stopped in a dangerous spot.

Explanation: Check the certification label for the date of manufacture. On older towed units, look under the vehicle for the ECU and wheel speed sensor wires.

Explanation: Tractors, trucks, and buses have the lamp on the instrument panel. Trailers have it on the left side, front or rear corner.

Explanation: Newer systems show the lamp briefly for a bulb check. On older systems it could stay on until you are driving over 5 mph.

Explanation: ABS adds to your normal brakes. It does not increase or decrease normal braking ability and works only when wheels are about to lock up.

Explanation: Allow time for at least 100 psi to build in both systems, watching both gauges (or needles).

Explanation: With one system low, either the front or rear brakes are not fully operating, so it takes longer to stop. Stop safely and have it fixed.

Explanation: If the compressor is belt-driven, check that the belt is in good condition and properly tight.

Explanation: Release the parking brakes so the slack adjusters can move, with the wheels chocked so the vehicle cannot roll.

Explanation: Out-of-adjustment brakes are the most common roadside inspection problem, so check the slack adjusters.

Explanation: Vehicles built since 1994 have automatic slack adjusters, which still must be checked.

Explanation: Manually adjusting an automatic adjuster usually masks a mechanical problem. Use it only as a temporary emergency fix and get the vehicle repaired soon.

Explanation: Drums or discs must not have cracks longer than half the width of the friction area.

Explanation: Single air systems should build from about 50 to 90 psi within 3 minutes. Dual systems should build from 85 to 100 psi within 45 seconds.

Explanation: Move forward at about 5 mph and brake firmly. Note any pulling to one side, unusual feel, or delayed stopping.

Explanation: Control the pedal for a smooth stop and do not push in the clutch until engine rpm is close to idle. When stopped, select a starting gear.

Explanation: Hydraulic brakes work instantly, but air brakes take 1/2 second or more for the air to reach the brakes. This delay is brake lag.

Explanation: Perception distance + reaction distance + brake lag distance + braking distance = total stopping distance.

Explanation: Overheating changes the lining chemically, reducing friction, and expands the drums so the shoes must travel farther and press with less force.

Explanation: Brakes out of adjustment quit doing their share first, overloading the others. Brakes can go out of adjustment quickly when hot, so check often.

Explanation: Get into the correct low gear first. The brakes only supplement engine braking on a long or steep downgrade.

Explanation: Apply the brakes hard enough to feel a definite slowdown, and release them when speed is about 5 mph below your safe speed.

Explanation: Each application, from your safe speed down to about 5 mph below it, should last about 3 seconds.

Explanation: Do not brake until you reach 40 mph, then slow to 35 mph and release. Repeat until the bottom of the grade.

Explanation: Spring brakes do not act on all axles, so a heavy vehicle stops slowly. Stop while you still have enough air to use the foot brake.

Explanation: Wet brakes can freeze if set in freezing weather. Dry them by braking lightly in a low gear, and use wheel chocks on a level surface.

Explanation: Tractor-only ABS helps keep steering control, but watch the trailer and ease off the brakes if it starts to swing out.

Explanation: The one exception to normal braking: with working ABS on all axles, you can fully apply the brakes in an emergency stop.

Explanation: A failed warning means you could lose air without knowing it, which can cause sudden emergency braking in a single-circuit system.

Explanation: The emergency system is not separate hardware. It uses parts of the service and parking brake systems.