#1. Should you pump (keep pressing and releasing) air brakes?
Each time you release the brakes, some compressed air leaves the system and must be replenished by the air compressor. If you keep pressing and releasing the brake pedal, air may leave the system faster than the air compressor can replenish it. The air pressure may drop to the point that the low air pressure warning signal will come on.
#2. Which of the following is an advantage of wedge brakes over S-cam drum brakes?
In a wedge braking system, the brake chamber directly faces the brake drum. Compared with S-cam drum brakes, wedge brakes lack the exposed linkage of the pushrod, clevis pin and external slack adjuster. (Most wedge brakes have internal automatic adjusters.) Today, most commercial vehicles have disc brakes or S-cam drum brakes. However, wedge brakes are still favored for some military and other applications for which the lack of exposed linkage is advantageous on rough, corrosive, or dangerous ground.
#3. When can you adjust automatic slack adjusters?
Once properly installed, automatic slack adjusters shouldn’t need manual adjustment. If an automatic slack adjuster strokes beyond the maximum allowed, this usually indicates that there are other brake problems that need to be repaired by a qualified brake service mechanic. If you are not so qualified, then don’t attempt to adjust an automatic slack adjuster except in an emergency in which your brakes have failed on the road
#4. To test the air brakes, the vehicle should be parked
Some tests require you to release the parking (spring) brakes. Before you start the tests, park on a level surface to reduce the chance of the vehicle rolling after the parking brakes are released. For added safety, chock the wheels whenever you must exit the vehicle.
#5. 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.)
#6. With the engine idling at _________, air pressure must rise from 85 to 100 psi within two minutes.
To test the air pressure build-up time, start by reducing the pressure to below 552 kPa (80 psi). If the vehicle has a trailer attached, ensure the trailer supply valve is closed (pulled out). With the engine idling at 600–900 rpm, the air pressure must rise from 587 kPa (85 psi) to 690 kPa (100 psi) within two minutes.
#7. A tractor-trailer with one trailer will fail the air-loss rate test if the loss rate exceeds
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.
#8. When you use the free stroke (pry bar) method, the pushrod should not move more than
To measure pushrod stroke with the free stroke (pry bar) method, use a pry bar to pull the pushrod out of the brake chamber. It should only move ½ – ¾ inch (13 mm – 19 mm). If the pushrod moves more than that, the brake needs adjustment.
#9. While you’re charging the trailer braking system, you find that the trailer brakes do not release. What are some possible causes?
If you cross-connect the air lines, supply air will be sent to the trailer service line instead of the trailer air reservoirs. Reservoir air will not be available to release the trailer spring brakes (parking brakes). And when you press the brake pedal, service air won’t flow to the trailer brake chambers, so the service brakes won’t work either.
#10. Drivers must hold an industrial air brake endorsement if they will be __________ on industrial roads.
Very few drivers require an industrial air brake endorsement. You need an industrial endorsement only if you’ll be operating unlicensed vehicles that are equipped with air brakes and are used to transport natural resources on industrial roads. (An industrial road is any road that exists for the transportation of natural resources, machinery, materials or personnel. Industrial roads include all company bridges, wharves and log dumps.) You don’t need an industrial air brake endorsement to operate skidders, loaders, graders or yarders on industrial roads.
#11. How often should the air tanks be drained?
Moisture that accumulates in the air tanks can freeze overnight, and the ice can stop air brake valves from working properly. To prevent this, drain the air tanks at the end of each workday
#12. 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.
#13. When you press the brake pedal, the valve underneath the brake pedal ________ to apply the service brakes.
To enable the driver to apply the service brakes by pressing the brake pedal, a foot (treadle) valve is located underneath the pedal. This valve has two supply (input) ports (one from the primary air tank and one from the secondary air tank), two delivery (output) ports, and an exhaust port. When the driver presses the brake pedal, the foot valve opens the delivery ports, and compressed air leaves the delivery ports to activate the service brakes. When the driver releases the brake pedal, the delivery ports close. The air in the brake chambers and hoses is exhausted to the atmosphere, releasing the service brakes. (Note: In many modern CMVs, only some of that air is exhausted through the foot valve’s exhaust port. Some of it is exhausted through the exhaust ports of other valves.)
#14. 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.
#15. The governor cut-in pressure is normally _________ below the cut-out pressure.
Governor cut-in pressure is normally 20–25 psi below the cut-out pressure.
#16. The minimum governor cut-in pressure is
The minimum governor cut-in pressure is 552 kPa (80 psi).
#17. Minimum governor cut-out pressure is
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Governor cut-out pressure should be set somewhere in the range 724–931 kPa (105–135 psi). Usually, it’s 125 psi (862 kPa)
#18. The most common cause of loss of air between the tractor and the trailer is
Glad hand couplers have seals made of rubber or polyurethane, which are designed to prevent air from escaping. Loss of air between the tractor and the trailer is commonly caused by a defective glad hand seal in the service (control) or supply couplers.
#19. During your brake tests, you find that the brakes are not operating safely. What should you do?
If there is any indication of bad brakes, don’t drive the vehicle any farther. Pull over, park, and secure the vehicle. Report the defect so it can be repaired before the vehicle is driven again
#20. In a vehicle equipped with air brakes, the most common cause of loss of braking power is
The most common cause of loss of braking effort is brakes that are out of adjustment. Federal and provincial laws require you to check manual and automatic slack adjusters daily, during the pre-trip inspection. This includes checking pushrod travel.
#21. To test the tractor protection valve, disconnect both trailer air lines and
If a severe loss of air pressure occurs, the tractor protection valve should close to prevent any more compressed air from leaving the tractor, so the tractor service brakes can still work. To test the valve, disconnect both air lines. Make a foot brake application. No air should be coming out of either the service line or the supply line.
#22. An improper angle between the slack adjuster and brake chamber pushrod when the brakes are applied can be corrected by
The slack adjuster can be adjusted to be at about a 90-degree angle to the pushrod. However, a 90-degree angle is NOT a reliable indication that the brake is properly adjusted; the angle can be as much as 10 degrees greater or less, depending on the model of brake adjuster. Adjust the slack adjuster so that pushrod travel is correct for the type of brake chamber.
#23. What is the purpose of the hand valve?
The trailer hand valve control (also known as a spike, a trolley valve, or a Johnson bar) controls the service brakes only on the trailer, not the tractor. You can use it to test the trailer service brakes independently of the tractor service brakes, such as after coupling your tractor to a trailer and during your pre-trip inspection of the combination vehicle.
#24. in a drum brake, the brake lining is fastened to
In a drum brake, a brake lining is attached to each brake shoe. When the service brakes are applied, the brake shoes force the linings against the inner surface of the spinning drum. Friction between the linings and the drum slows the rotation of the drum
#25. The spring brakes will start to apply when the air pressure falls below
If air pressure falls below 414 kPa (60 psi), the spring brakes will begin to apply automatically.
