Washington CDL: Air Brakes Practice Test

Not quite. The answer is Do all of the above.

To ensure that your vehicle will be safe when you leave it unattended, apply the parking brakes or chock the wheels. Closing the windows and removing the keys are simple safety tips for any vehicle, even a passenger car.

Not quite. The answer is 3 psi in one minute.

When testing for an air leak, make sure the system is fully charged, turn off the engine, fully apply the foot (service) brake, and hold the brake for one minute. The air pressure should not drop more than 3 psi for a single vehicle or more than 4 psi for a combination vehicle. If it does, check for an air leak somewhere.

Not quite. The answer is all of the above.

Spring brakes are an important backup system: powerful springs that automatically apply the brakes if they sense that air pressure has been lost for some reason, such as a leak. They will also apply the brakes if the psi gets too low, although ideally, you should take control of your brakes before that happens.

Not quite. The answer is controls the air pressure to operate the brakes.

When you press the brake pedal in a CMV equipped with air brakes, the foot valve underneath the pedal opens to allow compressed air to flow to the brake chambers. The brake chambers exert mechanical force to operate the foundation brakes.

Not quite. The answer is They work as a supplement to the braking effect of your engine.

When you drive down long, steep downgrades, the braking effect of your engine is what's most important, and your brakes are only a supplement to being in the proper low gear. They cannot compensate for your vehicle being in an unsafe high gear without putting too much pressure on your brakes.

Not quite. The answer is 60 psi.

A low pressure warning signal visible to the driver must come on before the pressure in the service air tank falls below 60 psi (or one-half the air compressor governor cutout air pressure on older vehicles).

Not quite. The answer is Never

It used to be thought that applying the steering axle brakes on a slippery surface might cause a front-wheel skid. However, more recent tests have shown that front-wheel braking is good under all driving conditions. Front-wheel skids from braking are unlikely even on ice. Therefore, always have front-wheel braking available. If you're driving an old truck equipped with a front brake limiting valve, leave the control set at "normal" to have normal stopping power. The front-wheel brakes provide about 12% of the total stopping power of a loaded combination vehicle.

Not quite. The answer is forces the brake shoes against the inside of the brake drum.

When the S-cam rotates, it forces the brake shoes apart and against the inside of the brake drum. Friction will slow the rotation of the wheel.

Not quite. The answer is Only if it's not working properly

Once properly installed and lubricated, an automatic slack adjuster should not need manual adjustment. If it does, it may be malfunctioning or there may be a problem with the foundation brake itself. Find the problem and correct it.

Not quite. The answer is The heavier the vehicle or the faster it's traveling, the more heat the brakes have to absorb to stop it.

Brakes use friction to convert the kinetic energy of your vehicle into heat. A vehicle's kinetic energy is directly proportional to its weight and directly proportional to the square of its speed. If you increase your speed, your brakes will have to absorb much more heat to bring your vehicle to a stop.

Not quite. The answer is takes longer than hydraulic fluid to flow through the lines.

With hydraulic brakes, hydraulic fluid is always in the system, so the brakes will be applied instantly when you depress the brake pedal. With air brakes, the compressed air must flow through a complex series of lines, valves, etc., to reach and fill the brake chambers before the brakes can begin to slow your vehicle. This "brake lag" can take at least a half second.

Not quite. The answer is one-half

During your walk-around inspection, check the brake drums or brake discs to be sure that no cracks are longer than half the width of the friction area.

Not quite. The answer is 32 feet

55 mph = 81 feet per second. Air brake lag typically lasts about 0.4 seconds. Therefore, the lag distance equals 81 feet per second × 0.4 seconds = 32 feet.

Not quite. The answer is vehicles with air brakes.

A low air pressure warning signal is required on vehicles with air brakes. The warning is usually a red light. A buzzer may also come on. Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops between the dangerous levels.

Not quite. The answer is that you are applying to the brakes.

If your vehicle is so equipped, the application pressure gauge will show how much air pressure you are applying to the brakes. If you have to apply more pressure than before to achieve the same braking effect, your brakes are fading (becoming less effective).

Not quite. The answer is It is exhausted from the system.

Each time you release the brakes, some compressed air leaves the system and must be replenished by the air compressor.

Not quite. The answer is slack adjuster; brake camshaft

To apply the S-cam drum brakes, air is let into each brake chamber. Air pressure pushes the pushrod out, moving the slack adjuster, thus rotating the brake camshaft. As the S-cam on the camshaft rotates, it forces the brake shoes against the inside of the brake drum.

Not quite. The answer is Clockwise until the bolt stops turning

Adjusting an automatic slack adjuster is an emergency procedure that you should perform only if your brakes have failed on the road. Most automatic slack adjusters have a hexagonal (six-sided) adjusting bolt. For this type of slack adjuster, first turn the bolt clockwise until it stops turning. If the slack adjuster has a square adjusting bolt, first turn the bolt counterclockwise until it stops turning. Then with either type of bolt, back it off about half a turn in the opposite direction.

Not quite. The answer is the service brake valve and the parking brake valve.

A spring parking brake can exert a force of up to 2,000 pounds, and a Type 30 service brake chamber can exert a force of 3,000 pounds or more. If both forces were applied at the same time (which is called compounding the service and parking brakes), the combined force could damage parts of the braking system. To prevent this, most modern commercial vehicles employ an anti-compounding valve. One input port of this valve is connected to the service brake valve (foot valve), and another input port is connected to the parking brake valve (yellow knob). Suppose you set the parking brakes by pulling out the yellow knob. That valve will send air to the anti-compounding valve. If you also press the brake pedal, the foot valve will also send air to the anti-compounding valve. Then the anti-compounding valve will partially release the parking brakes by the same amount of force that the foot valve has applied.

Not quite. The answer is brake lag.

When you depress the brake pedal on a vehicle with air brakes, there will be a lag of about a half second before the air flows through the lines to reach the brakes. This will add to your total stopping distance.