Air brakes stop a heavy vehicle with compressed air instead of brake fluid. The Air Brakes test asks how the system builds, stores and uses that air, and what you check before you drive.
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air brakes
пневматичні гальма
Brakes that use compressed air instead of fluid to stop heavy vehicles.
An engine-driven compressor fills tanks with air. When you press the pedal, that air pushes the brakes on. Air brakes are really three systems: service, parking and emergency.
Tested:what air brakes use to work, which parts belong to the system, and why a separate air brakes test exists.
The engine-driven pump that fills the air tanks with compressed air.
The engine turns the compressor through gears or a belt. Some compressors have their own oil supply; if yours does, check the oil level before driving. At operating rpm a dual system should build from 85 to 100 psi within 45 seconds.
Tested:what the compressor does, how it is driven, what to check on it, and what holds air in the tanks if it fails.
When tank pressure reaches the cut-out level, the governor stops the compressor. When pressure drops to the cut-in level, it starts the compressor again.
Tested:what the governor controls and how you check that it cuts in and cuts out correctly.
A reservoir that stores compressed air for the brake system.
The tanks hold enough air to brake several times even if the compressor stops. Water and compressor oil collect at the bottom and can freeze, so the tanks must be drained.
Tested:why tanks must be drained, what collects in them, and how trailer and dolly tanks are filled.
A valve that lets air out when tank pressure gets too high.
It protects the tank and the rest of the system and is usually set to open at 150 psi. If it ever opens, something is wrong and a mechanic needs to look.
Tested:questions ask at what pressure the safety valve opens and what it means when it releases air.
The rod the brake chamber pushes out to move the slack adjuster.
How far the pushrod travels shows brake adjustment. If the slack adjuster moves more than about one inch where the pushrod attaches, the brake needs adjusting.
Tested:what the pushrod does when air enters the chamber and how its travel relates to brake adjustment.
The lever between pushrod and S-cam that takes up brake slack.
Automatic slack adjusters adjust themselves but still must be checked. If one moves more than about one inch when you pull hard on it, the brake needs adjusting. Brakes out of adjustment are what roadside inspectors find most often.
Tested:how to check slack adjusters, how much movement means they need adjusting, and whether automatic ones may be adjusted by hand.
An S-shaped cam that pushes the brake shoes against the drum.
The S-cam drum brake is the most common foundation brake on heavy vehicles. When you release the pedal, the cam turns back and a spring pulls the shoes away.
Tested:what the S-cam does, why it has that name, and which foundation brake type is most common.
Brakes that squeeze pads against a spinning disc to slow the wheel.
In air disc brakes a power screw replaces the S-cam and clamps the disc like a big C-clamp. They are less common than S-cam drum brakes on heavy trucks.
Tested:disc brakes appear mostly as an answer option; questions ask how they differ from S-cam drum brakes and which brake types are common.
Brakes applied by powerful springs and held off by air pressure.
While you drive, air holds the springs back. If pressure leaks down to about 20 to 45 psi, the springs put the brakes on by themselves. Spring brakes serve as parking and emergency brakes.
Tested:what spring brakes are, what holds them off, at what pressure they apply, and how to check they come on automatically.
The brake that holds the vehicle still when parked.
On an air-brake truck you pull the yellow diamond knob to set it and push it in to release. Air can leak away, so the parking brake must hold by mechanical force.
Tested:how the parking brake control works, when you must use it, and when you should not set it.
The brake system that stops the vehicle if the service brakes fail.
It uses parts of the service and parking systems. On most trucks the spring brakes do this job. On a trailer, losing air in the emergency line applies them.
Tested:what applies the trailer emergency brakes, what makes them come on suddenly, and what must hold them on.
Two separate air brake systems that share one set of controls.
One system usually works the rear brakes and the other the front, each with its own tanks and lines. Before driving, wait until both halves show at least 100 psi. If one loses pressure you still have some braking, but a longer stop.
Tested:what a dual system is, what pressure both halves need before driving, and what to do if one half shows low pressure.
The delay before air brakes act after you press the pedal.
Air needs time to travel through the lines to the brake chambers. Hydraulic brakes act at once; air brakes take about half a second or more. At 55 mph that delay adds about 32 feet to your stop.
Tested:what brake lag is, why air brakes have it, and how much distance it adds at highway speed.
Loss of braking power because the brakes have overheated.
Heat changes the lining and expands the drum, so the shoes grip less. Fade comes from using the brakes too much instead of the engine on a downgrade. Brakes out of adjustment make the others overheat.
Tested:what causes brake fade, how to avoid it on a long downgrade, and what adjustment has to do with it.
Braking fully, releasing when the wheels lock, then braking fully again.
Wait until the wheels start rolling again before you press again, or the vehicle will not straighten out. Vehicles with an anti-lock braking system (ABS) are braked differently: keep firm pressure and let ABS work.
Tested:how to perform stab braking, how long to wait before reapplying, and which vehicles it is not for.
Stopping as quickly as possible while keeping the vehicle straight and steerable.
Hitting the brakes is the right response only if you have room to stop and brake correctly. Use controlled braking or stab braking. With ABS, brake hard and steer.
Tested:what emergency braking is, which two methods it uses, and what the emergency brake system uses when brakes fail.
A computer system that keeps the wheels from locking during hard braking.
ABS senses a wheel about to lock and lowers brake pressure at that wheel. It does not make you stop shorter; it helps you keep steering control. A yellow lamp shows a fault.
Tested:what ABS does, how you can tell a vehicle has it, what the yellow lamp means, and how to brake with it.
A device that slows the vehicle without using the wheel brakes.
There are exhaust, engine, hydraulic and electric retarders. They work on the drive wheels when you lift off the accelerator. Turn them off on wet, icy or snowy roads because the drive wheels can skid.
Tested:what retarders do, which types exist, when to switch them off, and where their noise may be restricted.
A retarder that uses the engine itself to slow the vehicle.
Drivers often call it a Jake brake. Be in a low gear before a long downgrade so the engine does most of the slowing. Like any retarder, it acts on the drive wheels only, so switch it off on slippery roads.
Tested:how to get the most from an engine brake and how engine braking fits into downgrade technique.
A valve that saves the tractor's air if the trailer breaks away.
The trailer air supply knob in the cab opens and closes it. It also closes by itself when pressure drops to about 20 to 45 psi. When it closes, the trailer emergency brakes come on.
Tested:what the tractor protection valve does, what control operates it, and how to test that it closes.
The red knob that sends air to the trailer or shuts it off.
Push it in to supply the trailer with air. Pull it out to cut the air and set the trailer emergency brakes. It pops out on its own when pressure drops into the 20 to 45 psi range.
Tested:what the control does, what else it is called, which way you move it, and when it pops out.
The air line that supplies the trailer and controls its emergency brakes.
It is usually red and is also called the supply line. If it loses pressure, the trailer emergency brakes come on and the tractor protection valve closes.
Tested:questions ask the line's two jobs, its color, its other name, and what happens when it loses pressure.
The couplers that join the tractor's air lines to the trailer's.
Each one has a rubber seal; clean it before connecting. Press the two seals together at a right angle and turn to lock. With no trailer attached, park them on dummy couplers or lock them together.
Tested:what glad hands connect, how to join them, why colors help, and what to do with them when not pulling a trailer.
How fast air pressure drops when the engine is off.
You test it twice: with the brakes released, and with the pedal held down for a minute. The limits differ for a single vehicle and a combination.
hydraulic brakes
гідравлічні гальма
Brakes that use fluid pressure, not air, to apply the brakes.
Cars and lighter trucks have them. They work the moment you press the pedal, so there is no brake lag. Test for leaks by pumping and then holding the pedal.
Tested:how to test hydraulic brakes for leaks, what a sinking pedal means, and what to do if they fail.
