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Air Brake Mock

#1. What function does the alcohol evaporator perform?

When the air compressor runs, an alcohol evaporator puts alcohol vapor into the air brake system to help prevent water in the lines or valves from freezing. Ice in the lines or valves can cause them to stop working.

#2. How often should you check brake chamber pushrod travel?

According to the Canada National Safety Code, pushrod travel must be checked daily. Do it as part of your pre-trip inspection

#3. As part of your pre-trip inspection of a combination vehicle with air brakes and one trailer, you should make sure that the air-loss rate per minute is not greater than

In British Columbia, the maximum acceptable leakage rates per minute are 28 kPa (4 psi) for single vehicles (buses, straight trucks and bobtail tractors) and tractor-trailers with one trailer, and 41 kPa (6 psi) for tractor-trailers with two trailers. If the leakage rate is higher, check for leaks and correct them.

#4. You're driving a vehicle equipped with a dual air brake system. The low air pressure warning signal comes on, but one system still has normal pressure. What should you do?

Virtually all large commercial vehicles today are equipped with a dual-circuit air brake system for greater safety. A dual air brake system has two separate air brake systems, both controlled by the same set of brake controls. Each system has its own air tanks, hoses, lines, etc. The primary system typically operates the regular brakes on the rear axle or axles. The secondary system operates the regular brakes on the front axle (and possibly one rear axle). If one system loses air pressure, you’ll still be able to stop your vehicle by using the brakes controlled by the other system, but your vehicle’s braking distance will increase, which isn’t good. Better to bring your vehicle to a safe stop and get the brakes repaired.

#5. The air-loss rate test can be performed only when

To perform the air-loss rate test, first secure the vehicle and release the spring brakes. Next, ensure that air pressure is within its normal operating range. Now fully apply and hold the brake pedal to keep sending compressed air to the service brakes. Watch your air pressure gauges and see what happens to the service tank air pressures.

#6. It is important to keep compressed air as dry as possible because

Water and oil from the compressor can collect at the bottoms of the air tanks. If the air tanks are not drained regularly, the water can freeze and cause brake failure. Some air brake systems include an alcohol evaporator to prevent ice from forming in the air brake system. (Alcohol has a lower freezing point than water.)

#7. In which of the following situations should a spring parking brake chamber be caged?

If the spring parking brakes have come on because of a loss of air pressure in the braking system, they may have to be released to tow or repair the vehicle. Without air pressure, the only way to release a spring brake is to manually insert a release bolt into the spring brake chamber to release the spring. This is called “caging” the spring

#8. You're driving a vehicle equipped with air brakes. If the low air pressure warning signal comes on, what should you do?

A low air-pressure warning signal indicates that reservoir air pressures have dropped below 60 psi (414 kPa), which means a serious air brake system safety hazard. Bring your vehicle to a safe stop as soon as possible and try to find the cause of the low pressure. If the air pressure drops much lower, your service brakes may fail completely.

#9. The maximum air pressure available for braking is determined by

Compressed air flows from the tanks to the brake chambers. The greater the air pressure in the tanks, the greater the air pressure at the brake chambers. The air compressor governor keeps the tank air pressure within a narrow range.

#10. When are water-cooled brakes used?

Many vehicles that operate on industrial roads are extremely heavy. Stopping these heavy vehicles puts a lot of pressure on their brakes, which can cause the brakes to overheat. When brakes overheat, the drums expand away from the brake linings, leading to brake fade (loss of braking effectiveness). Some of these vehicles use water directed at the brake drums to help prevent brake drum overheating

#11. If your vehicle is equipped with automatic slack adjusters, why must you fully apply the service brakes regularly?

Automatic slack adjusters adjust themselves to compensate for excessive pushrod stroke. If you only make normal service brake applications of 40 – 50 psi (276 – 345 kPa), the automatic slack adjusters will maintain adjustment only for those shorter strokes, not for long strokes from full brake applications of 100 psi (690 kPa). Therefore, to keep automatic slack adjusters properly adjusted, make full brake applications regularly. For example, try to fully apply the service brakes every time you stop at a red light or stop sign

#12. If you uncouple and park an old, single-circuit trailer, you should do which of the following?

A single-circuit trailer has only one braking circuit. And an old trailer might not have a backup braking system (such as spring brakes) either. Hence, if air pressure is slowly lost in that one braking circuit, the trailer brakes will release and the trailer might roll away. To prevent this, block (chock) the trailer wheels as a precaution.

#13. You're driving a tractor-trailer. If you're applying a pressure of 30 psi to the trailer brakes with the foot valve, you can apply a greater pressure at the same time by also using

A two-way check valve allows compressed air from two different sources to flow through the same air line. The valve will select the source with the higher pressure. In this case, the foot valve and hand valve are connected to two-way check valves that in turn lead to the tractor protection valve, and from there to the control (service) line. For example, if the foot valve is sending compressed air at 30 psi (207 kPa) and the hand valve is sending compressed air at 40 psi (276 kPa), then compressed air at 40 psi (276 kPa) will be sent out to the trailer brakes.

#14. You're driving a vehicle equipped with an automatic front-wheel limiting system (ratio valve). If you brake hard during an emergency stop, the air pressure delivered to the front brakes will be approximately _____ of the air pressure to the rear brakes.

The front-brake limiting effect is greatest for brake application pressures below 40 psi (276 kPa). At brake application pressures over 60 psi (414 kPa), there is no limiting effect; the air pressure delivered to the front brakes equals that delivered to the rear brakes.

#15. In an air brake system, spring brakes are intended for

Trucks and buses must have parking and emergency brakes in addition to the service brakes. The parking and emergency brakes must be held on by mechanical force (because air pressure can leak away). Spring brakes are usually used to meet these needs.

#16. In a tractor-trailer, an anti-compounding valve is connected to

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 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 button). Suppose you have set the parking brakes by pulling out the yellow button, causing that valve to send air to the anti-compounding valve. Then if you press the brake pedal, causing the foot valve to also send air to the anti-compounding valve, the anti-compounding valve will partially release the parking brakes by the same amount of force that the foot valve has applied.

#17. A brake shoe is not a part of

In a disc brake, a caliper forces two brake pads against the disc (also known as a rotor). The resulting friction slows the wheel

#18. Assuming good traction and constant vehicle weight, if you double your speed, your vehicle's braking distance will be

The kinetic energy of a moving object increases as the square of the object’s speed. A vehicle travelling at 80 km/h has four times the kinetic energy of a vehicle travelling at 40 km/h. Brakes use friction to convert the kinetic energy of a moving vehicle into heat. Thus, if you double your speed, it will take four times as long for your brakes to stop your vehicle.

#19. The spring brake test is passed if the vehicle

To test the spring brakes, first remove any wheel chocks. Set the parking (spring) brakes by pulling out the yellow dashboard knob. Then gently apply engine power in low gear. The parking brakes should keep your vehicle from moving.

#20. Before inspecting your air brakes, you should

Before you start to test the air brake system, park your vehicle on level ground to prevent it from rolling. Apply the parking (spring) brakes and put wheel chocks or blocks in place to secure the vehicle. Note that some tests of the air brake system will require you to release the parking brakes, which is why it’s so important to block or chock the wheels as well.

#21. When draining the air tanks, you observe excessive oil coming out of the supply tank. Which of the following is most likely to be faulty?

Most modern air compressors are lubricated with the engine’s own oil supply. It’s normal for an air compressor to pass a trace amount of this engine oil. However, if the air dryer or supply tank has been contaminated with excessive oil, it could mean that the compressor is faulty and should be replaced.

#22. Air compressors can either be driven by the engine belt and pulleys or

Air compressors used to be driven by the engine belt and pulleys. But today’s modern air compressors are mounted directly on the engine and driven by gears. Some models are even lubricated with the engine’s oil supply

#23. What is the purpose of the governor?

A typical air compressor can produce air pressures of up to 500 psi (over 3,000 kPa), far higher than required for safe operation of the brake system. Therefore, an air compressor governor is employed to keep tank pressures within a relatively narrow range. When tank pressure rises to the “cut-out” level (typically about 862 kPa or 125 psi), the governor prevents the compressor from pumping more air into the tanks. When the tank pressure falls to the “cut-in” level, the governor allows the compressor to pump air into the tanks again. The cut-in level is typically 138–172 kPa (20–25 psi) below the cut-out level.

#24. What does an alcohol evaporator do?

In cold weather, moisture in the air brake system can freeze. Ice in the lines or valves can cause them to stop working. To help prevent this, the air brake system may include an alcohol evaporator. When the air compressor runs, the alcohol evaporator puts alcohol vapor into the air brake system to help prevent water in the lines or valves from freezing. (Alcohol has a lower freezing point than water.) Use only the type of alcohol that is recommended by the evaporator’s manufacturer (typically some form of methanol).

#25. In British Columbia, when you test the system air-loss rate of a bus or straight truck, the maximum acceptable leakage rate is 4 psi within

In British Columbia, the maximum acceptable leakage rates per minute are 28 kPa (4 psi) for single vehicles (buses, straight trucks and bobtail tractors) and tractor-trailers with one trailer, and 41 kPa (6 psi) for tractor-trailers with two trailers. If the leakage rate is higher, check for leaks and correct them.
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