Air Brake Mock
#1. What is the purpose of a relay valve?
Relay valves are used to reduce the lag time, both when the brakes are applied and when they’re released. Relay valves are remote-controlled air valves that respond to a control signal from the foot valve. These valves are located near the brake chambers that they operate, and they receive a large volume of air from the service reservoirs. When the brake pedal is pressed, the control signal directs the relay valve to send air to the brake chambers. When the brake pedal is released, the air from the brake chambers is exhausted from a port in the nearby relay valve instead of having to flow back to the foot valve to be exhausted there. Only the air in the control line must flow back to the foot valve.
#2. To “tug test” the trailer service brakes after you couple your tractor to the trailer, you should apply
After coupling your tractor to the trailer, use the hand control valve to test the action of the trailer service brakes. This valve applies only the trailer service brakes, not the tractor service brakes. First, release the entire vehicle’s parking brakes. Next, slowly pull away and use the hand control valve to apply the trailer brakes to check their operation.
#3. How can you make sure that each air tank is securely attached to the vehicle?
To check that each air tank is securely mounted, look at it to see whether it’s loose, and then pull on it to see whether you can pull it loose. Also check the security of the mounting brackets and hardware that hold the air tank to the vehicle
#4. What does the low air pressure warning device warn you about?
Vehicles with air brakes are equipped with a low air pressure warning device. It will activate a red warning light on the dashboard if air pressure in the primary or secondary air tank has fallen to 414 kPa (60 psi) (although some manufacturers’ systems activate at higher pressures). If this happens, you should brake safely to a stop and repair the problem before the air pressure drops to the point that the service brakes won’t work properly. Note: On older vehicles, a “wig wag” (mechanical arm) may activate instead of a warning light.
#5. Polarized gladhands on a tractor
To prevent cross-connection of the supply and service lines, polarized gladhands and polarized trailer couplers are now available. The supply gladhands and couplers are shaped slightly differently from the service gladhands and couplers, making cross-connection impossible. However, polarized gladhands can still be connected to universal (non-polarized) trailer couplers, so cross-connection in this situation is still possible.
#6. The hand valve control (“spike”) applies
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.
#7. In the air service circuit, there is a double (two-way) check valve connected to the primary and secondary service tanks. What does this valve do?
The parking control valve (yellow button) in the dashboard is supplied with air from a double (two-way) check valve. This check valve supplies air (often called blended air) from the service tank with the highest pressure. If air pressure is lost in either the primary or secondary service circuit (but not both), the parking control (yellow button) will still work properly with air from the other circuit, and the spring parking brakes will stay released.
#8. An alcohol evaporator or air dryer can
In cold weather, moisture that has collected in the supply tank can freeze, and the ice can stop air brake valves from working properly. Two different devices can help alleviate this problem. An air dryer removes much of the moisture from the compressed air before it enters the supply tank. An alcohol evaporator puts alcohol vapour into the air to reduce the likelihood of freezing. (Alcohol has a lower freezing point than water.)
#9. A brake lining that has become contaminated with oil or grease indicates a problem with the
A brake lining that has become contaminated with oil or grease indicates a problem with the wheel. For example, oil can leak from a bad wheel seal. As the wheel spins, some of the leaking oil may be splashed onto the brake lining.
#10. 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.
#11. Why are drivers required to complete a trip inspection report?
Under Division 37 of the British Columbia Motor Vehicle Act Regulations, every carrier must require each of its drivers to submit a trip inspection report. A driver who fails to do so may be fined.
#12. 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.
#13. If there is a loss of pressure in the trailer supply line, you would expect the trailer brakes to be applied and
The tractor protection valve is typically located behind the cab. Both the supply (emergency) and service (control) air lines pass through the tractor protection valve on their way to the trailer. (The valve has intake and output ports for each line.) In the event of a severe loss of air pressure in the supply line (perhaps because the trailer separated from the tractor), the tractor protection valve will close to prevent compressed air from leaving the tractor, so the tractor service brakes will still be able to work. It will also let the air out of the trailer supply line, which will activate the trailer emergency brakes to stop the trailer.
#14. Where is the air dryer located?
The supply (“wet”) tank tends to get most of the moisture because moisture condenses there as the hot air from the compressor cools. To help alleviate this situation, the air dryer dries the air from the compressor before it enters the supply tank. Note: Today’s air dryer technology is so effective that a “wet” tank may not be needed anymore. In a modern Air Dryer Integrated System (ADIS), air from the air dryer goes directly into the primary and secondary service (“dry”) tanks.
#15. Where is the air pressure gauge for the service tanks?
The air pressure gauges for the service tanks are located on the dashboard. For a dual service brake system, there will either be a separate gauge for each service tank or a single gauge with two needles, one for each service tank.
#16. Which of the following are advantages of a dual air brake system?
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.
#17. 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.
#18. Assuming good traction and constant vehicle speed, if you double your vehicle’s weight, your vehicle’s braking distance will be
The kinetic energy of a moving object is directly proportional to the object’s weight. A vehicle that is twice as heavy has twice the kinetic energy. Brakes use friction to convert the kinetic energy of a moving vehicle into heat. Thus, if you double your vehicle’s weight, it will take twice as long for your brakes to stop your vehicle.
#19. 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.
#20. How often should you inspect your vehicle?
All commercial vehicles that fall under the National Safety Code (NSC) must be inspected before they’re used each day (pre-trip inspections) and at the end of the final trip of each day (post-trip inspections). These inspections are required under the Motor Vehicle Act Regulations. Either you or another person specified by the carrier must perform these inspections.
#21. For inspecting the air brakes, when should you chock the wheels?
You should chock the wheels whenever the vehicle or a trailer might move on its own and hurt somebody: (1) if the cab will be unoccupied; (2) if you or someone else will be underneath the vehicle (say to inspect the brakes); and (3) when you must perform braking tests that require the brakes to be released; (4) when coupling or uncoupling a trailer.
#22. Assuming good traction and constant vehicle speed, if you double your vehicle’s weight, the braking power to stop your vehicle in the same distance will be
The kinetic energy of a moving object is directly proportional to the object’s weight. A vehicle that is twice as heavy has twice the kinetic energy. Brakes use friction to convert the kinetic energy of a moving vehicle into heat. Thus, if you double your vehicle’s weight, it will take twice as much braking power to stop your vehicle in the same distance.
#23. When you test the system air-loss rate, the maximum acceptable leakage rates per minute for a straight truck, a tractor pulling one trailer, and a tractor pulling two trailers are
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.
#24. Normally, who can 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. Don’t attempt to adjust an automatic slack adjuster except in an emergency in which your brakes have failed on the road
#25. 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
