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This lesson tells you about air brakes. If you want to drive a truck or bus with air brakes, or pull a trailer with air brakes, you need to read this section. If you want to pull a trailer with air brakes, you also need to read the lesson about Combination Vehicles.
NOTE: You only need to take a written and skills test on the air brake system if the vehicle you intend to drive has an air brake system and requires that you have a CDL to operate it. If you do not pass the air brake written test, or if you test in a vehicle without air brakes, you will have a restriction (L) placed on your CDL. If you later wish to remove the restriction, you need to pass the written test and complete a new skills test in a commercial vehicle with air brakes. Once you have passed a test in a vehicle with air brakes, the restriction is permanently lifted.
Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large, heavy vehicles; however, the brakes must be well maintained and used properly. Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.
The parts of these systems are discussed in greater detail below.
There are many parts to an air brake system. You should know about the parts discussed here.
The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a v-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.
The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the “cut-out” level (around 125 pounds per square-inch or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the “cut-in” pressure (around 100 psi), the governor allows the compressor to start pumping again.
Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.
Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types:
Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather.
Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.
Check the alcohol container and fill up as necessary, every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).
A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.
You put on the brakes by pushing down the brake pedal (also called the foot valve or treadle valve). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes won’t work.
Foundation brakes are used at each wheel. The most common type is the S-cam drum brake. The parts of the brake are discussed below.
Brake Drums, Shoes, and Linings: Brake drums are located on each end of the vehicle’s axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.
S-cam Brakes: When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the S-cam (so called because it is shaped like the letter “S”). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.
Wedge Brakes: In this type of brake, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber, or two-brake chambers pushing wedges in at both ends of the brake shoes. Wedge type brakes may be self-adjusting or may require manual adjustment.
Disc Brakes: In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster like S-cam brakes. But instead of the S-cam, a “power screw” is used. When the brakes are applied, the pressure of the brake chamber on the slack adjuster turns the power screw. This causes the caliper to clamp two brake pads against a rotor (like a C-clamp). Wedge brakes and disc brakes are less common than S-cam brakes.
All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system, or a single gauge with two needles. Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.
This gauge shows how much air pressure you are applying to the brakes (This gauge is not on all vehicles). Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.
A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one half the compressor governor cutout air pressure on older vehicles). The warning is usually a red light- it may be on the dash or near the visor overhead. A buzzer may also come on. Another type of warning found on some very old trucks and busses is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops below about 55 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 55 psi. The manual reset type must be placed in the “out of view” position manually, and it will not stay in place until the pressure in the system is above 55 psi. On large buses it is common for the low pressure warning devices to signal at 80-85 psi.
Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.
Some older vehicles (made before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked “normal” and “slippery.” When you put the control in the “slippery” position, the limiting valve cuts the “normal” air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front wheel braking is good under all conditions. Tests have shown front wheel skids from braking are not likely even on ice. Make sure the control is in the “normal” position to have normal stopping power.
Many vehicles have automatic front wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (55 psi or more application pressure). These valves cannot be controlled by the driver.
All trucks, truck tractors, and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force, (because air pressure can eventually leak away). Spring brakes are usually used to meet these needs. When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes.
A parking brake control in the cab allows the driver to let the air out of the brake chamber, which results in the spring applying the brakes (the linings will be forced against the drums). A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.
Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi). Do not wait for the brakes to come on automatically. When the low air pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away, while you can still control the brakes.
The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.
In newer vehicles with air brakes, you put on the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to evacuate the air from the brake chamber and put the parking brakes (spring brakes) on, and push it in to release them. On older vehicles, the parking brakes may be controlled by a lever. Use the parking brakes whenever you park.
Caution! Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure. Many brake systems are designed so this will not happen. But not all systems are set up that way, and those that are may not always work. It is much better to develop the habit of not pushing the brake pedal down when the spring brakes are on.
Trailer Brake Control Valves: In some vehicles a control handle on the steering column or dash board may be used to apply the trailer brakes only, gradually. This brake is known by different names. It is sometimes called a Modulating Valve, Trolley Valve, Johnson Bar, etc. It may be spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the trailer brakes come on. Do not use the modulating control valve to park a vehicle. Use the parking brake(s).
Dual Parking Control Valves: When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. The other valve is spring loaded in the “out” position. When you push the control in, air from the separate air tank releases the spring brakes so you can move the vehicle.
When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times. Therefore, plan carefully when moving, otherwise, you may be stopped in a dangerous location when the separate air supply runs out.
Truck tractors with air brakes built on or after March 1, 1997, and other air brakes vehicles, (trucks, buses, trailers, and converter dollies) built on or after March 1, 1998, are required to be equipped with antilock brakes. Many commercial vehicles built before these dates have been voluntarily equipped with ABS. Check the certification label for the date of manufacture to determine if your vehicle is equipped with ABS. ABS is a computerized system that keeps your wheels from locking up during hard brake applications. Vehicles with ABS have yellow malfunction lamps to tell you if something isn’t working.
In the case of towed units manufactured before it was required by the Department of Transportation, it may be difficult to tell if the unit is equipped with ABS. Look under the vehicle for the electronic control unit (ECU) and wheel speed sensor wires coming from the back of the brakes.
ABS is an addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only activates when wheels are about to lock up. ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.