Air Brakes

This section will inform 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 study this section.

Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles but the brakes must be well maintained and used properly. Air brakes are really three different braking systems in one. There is a service brake, a parking brake and an emergency brake.

The service brake system applies and releases the brakes when you use the brake pedal during normal driving. The parking brake system applies and releases the parking brakes when you use the parking brake control. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a break system failure.

There are many parts to an air brake system. You should know about those parts discussed here. The air compressor pumps air into the air storage tanks. 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 air compressor has its own oil supply, check the oil level before driving. The easiest way to spot the air compressor in the motor is to look for the air governor. It is usually a silver cylinder with a black cap on top.

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 PSI, the governor stops the compressor from pumping air. When the tank pressure falls to the cutting pressure, around 100 PSI, the governor allows the compressor to start pumping again.

Here are some more air brakes system parts. 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.

Air tank drains. Compressed tanks usually have 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, manually-operated, by turning the quarter turn or by pulling a cable.

You must drain the tanks yourself at the end of each day. And the truck equipped with an automatic drain, the water and oil is automatically expelled. They may be equipped for manual draining as well. The automatic types are available with electric heating devices. These help prevent freeze up 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 brake stop working.

Check the alcohol container and fill up as necessary. Daily air tank drainage is still needed to get rid of water and oil unless the system has an automatic air drain valve. A safety release valve is installed in the first tank the air compressor pumps air into. 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. Just as in a car, you put on the brakes by pushing down the brake pedal. It is also called the foot valve or the throttle 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 properly.

Foundation brakes are used at each wheel. The most common type is the S-CAM drum brake. Each wheel has the following parts: 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 brake stop working. S-CAM brakes are the most common air brake system.

When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster that's twisting the brake cam shaft. 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.

Foundation brakes are used at each...there are two other types of air brake systems: A wedge brake and a disc brake. First, the wedge brake. In this type of brake, the brake chamber push rod pushes the wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drums. 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. In air-operated disc brakes, air pressure acts on a brake chamber in the slack adjuster like S-CAM brakes but instead of the S-CAM, a power screw is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper similar to a large sea clown. Wedge brakes and disc brakes are less common than S-CAM brakes.

There are also different types of gauges in an air brake system. All air brake vehicles 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. There is also an application pressure gauge. This gauge shows how much air pressure you're applying to the brakes.

This gauge is not on all vehicles. Increasing application pressure to hold the 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 that are 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 tank falls below 60 PSI or one-half the compressor governor cut out pressure on older vehicles.

The warning is usually a red light. A buzzer may also come on. In buses, it is common for the low pressure warning devices to signal at 80 to 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 have a front brake limiting valve and it's controlled 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 are 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. Test has shown that 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, 60 PSI or more application pressure. These valves cannot be controlled by the driver. All trucks, tractors and buses must be equipped with emergency braking and parking brakes. They must be held on by mechanical force. 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 release or set the parking brakes. A leak in the air brake system will cause the springs to put on the brakes. The spring brakes will come on, if air pressure drops to a range of 20 to 45 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 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 or parking brakes will work correctly. In newer vehicles, with air brakes, you put on the parking brakes using a diamond shaped, yellow push control knob. You pull the knob out to put the parking 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 but use 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 this 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 applied.

Modulating Control Valves: In some vehicles, a control handle on the dashboard may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring-loaded, so you have to feel for the brake in action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go, and hold it in place with a locking device.

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 that 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 brake for parking. The other valve is spring loaded in the out position.

When you push the control in, air from a separate air tank releases the spring brake so you can move. When you release the button, the spring brakes come on again. There is only enough air in a separate tank to do this a few times. Therefore, plan carefully when moving, otherwise, you may be stopped in a dangerous location when this separate air supply runs out.

Anti-lock brakes or ABS is a computerized system that keeps your wheels from locking up during the hard braking applications. ABS is in addition to your normal brakes. It does not decrease or increase your normal braking capabilities. 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 a hard braking. The Department of Transportation requires that ABS be on all truck tractors with air brakes built on or after March 1, 1997.

Other air brake vehicles such as trucks, buses, trailers and converter dollies built on or after March 1, 1998, hydraulically brake trucks and buses with a gross vehicle weight rating of 10,000 pounds or more, built on or after March 1, 1999. ABS will not allow you to drive faster, follow more closely or drive less carefully. ABS will not prevent power or turning skids, ABS should prevent brake-induced skids or jack-knives but not those caused by spinning the drive wheels or going too fast in a turn. ABS will not necessarily shorten stopping distance.

ABS will help maintain vehicle control but not always shorten stopping distance. ABS will not increase or decrease ultimate stopping power; ABS is an add-on to the normal brakes, not a replacement for them. ABS will not change the way you normally brake. Under normal braking conditions, your vehicle will stop as it always has. ABS only comes into play, when a wheel would normally have locked up because of over-braking. ABS will not compensate for bad brakes or poor performance. Remember, the best vehicle safety feature is still a safe driver. Drive so you never need to use your ABS. If you need it, ABS could help prevent a serious crash.

Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other systems operate the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer. The first system is called the primary system; the other system is called the secondary system.

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 PSI pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges or needles if the system has two needles in one gauge. Pay attention to the low air pressure warning light and buzzer. The warning light and buzzer should shut off when air pressure in both systems rises greater than 60 PSI.

The warning light and buzzer should come on before the air pressure drops below 60 PSI in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low in pressure, either the front or the rear brakes will not be operating fully. This means that it will take you longer to stop. Bring the vehicle to a safe stop and have the air brake system fixed.

Inspecting Your Air Brake System: There are more things to inspect on a vehicle with air brakes than one without them. Engine compartment checks. First identify the air compressor. One indication is to look for the air governor. It is usually on the driver side of the vehicle. If the air compressor is belt-driven, check the condition and tightness of the belt. The belt should be in good condition, not excessively loose, with no wears, tears or frays. During your walk-around inspection, check manual slack adjustors on S-CAM brakes.

Park on level ground and chuck the wheels to prevent the vehicle from moving. Turn off the parking brakes, so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can get to. If a slack adjuster moves more than about 1 inch, with a push rod attached to it, it needs adjustment. Vehicles with too much brake slack can be very hard to stop. Out of adjustment brakes are the most common problem found in roadside inspections. Be safe, check the slack adjusters.

Check brake drums or discs linings and hoses. Brake drums or discs must not have cracks longer than one-half the width of the friction area. Linings must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing. Check that the air hose is connected to the brake chambers to make sure they aren't cut or worn due to rubbing.

Some of the rules for air brake check were changed by the Government in July 2015. The new procedure is as follows. Parking Brake Check: With the truck in low, forward gear and the parking gear set, attempt to move the truck. If the truck does not move, the parking brake is functioning properly.

Service Brake Check: Brake pedal. Release the parking brake and move the vehicle forward at less than 5 miles per hour. Depress the brake pedal while gripping the steering wheel lightly. The truck should not pull to the right or left during braking. If it does, the brakes are out of adjustment.

Applied Air Loss Test: Turn the engine off, but the key on, depress the brake pedal and hold it down. Look at the air gauges, observe that there is no more than 4 PSI loss for Class A vehicle in 1 minute. Class B vehicles cannot lose more than 3 PSI.

Low Air Warning: Begin fanning the brake pedal until the air gauges read 60 PSI. The low air warning light and/or buzzer should come on. Continue fanning until the air pressure reaches 40 to 20 PSI, at which point the brake valves should pop out. Watch the valves, not the gauges.

Using Air Brakes: First normal stops. Push the brake pedal down, control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the button until the engine RPM is down, close to idle. When stopped, select the starting gear.

Braking with ABS: When you drive a vehicle with ABS, you should brake as you always have. In other words, use only the braking force necessary to stop safely and in control. Brake the same way regardless of whether you have ABS on the bus, tractor, the trailer or both. As you slow down, monitor your tractor and trailer and back off the brakes in order to control. There's only one exception to this procedure. If you drive a straight truck or combination with working ABS on all axles, in an emergency stop, you can fully apply the brakes.

If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there's enough distance to stop and you use the brakes correctly. You should brake in a way that will keep the vehicle in a straight line and allow you to turn if it becomes necessary. You can use a controlled braking method or the stab braking method.

The Controlled Braking Technique: With this method, you apply the brakes as hard as you can, without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make a larger steering adjustment or if the wheel is locked, release the brakes. Reapply the brakes as soon as you can.

The Stab Braking Technique: Apply your brakes all the way, release brakes when wheels lock up. As soon as the wheels start rolling again, apply the brakes fully. It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle won't straighten out. Don't jam on the brakes. Emergency braking does not mean pushing down on the brake pedal as hard as you can. That will only keep the wheels locked up and cause a skid. If the wheels are skidding, you cannot control the vehicle.

With the air brakes, there's an added delay. The time required for the brakes to work after the brake pedal is pushed, with hydraulic brakes, the brakes work instantly. However, with air brakes, it takes a little time, sometimes one-half second or more for the air to flow through the lines to the brakes, thus the total stopping distance for vehicles with air brake systems, is made up of four different factors: Perception distance, reaction distance, brake-lag distance and effective braking distance. That equals total stopping distance.

The air brake lag distance at 55 miles per hour on dry pavement adds about 32 feet. So, at 55 miles per hour, for an average driver, under good traction and braking condition, the total stopping distance is over 300 feet. This is longer than a football field. Brake fading or failure, you must go slow enough so your brakes can hold you back without getting too hot. If the brakes become too hot, they may start to fade.

This means that you'll have to apply them harder and harder to get the same stopping power. If you continue to use the brakes hard, they can keep fading until you cannot slow down or stop at all. Brakes are designed so that brake shoes or pads rub against the brake drums or discs. Brake increases heat but brakes are designed to take a lot of heat, however, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.

Brake fade is also affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be enough braking available to control the vehicle. Brakes can get out of adjustment quickly, especially when they're used a lot. Also, brake linings wear faster when they're hot, therefore, brake adjustment must be checked frequently.

Remember, the use of brakes on a long or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in a proper low gear, the following is the proper braking technique. Apply the brakes just hard enough to feel a definite slow down. When your speed has been reduced to approximately 5 miles per hour below your safe speed, release the brakes.

This brake application should last for about 3 seconds. When your speed has increased to your safe speed, repeat steps one and two. For example, if your safe speed is 40 miles per hour, you'll not apply the brakes until your speed reaches 40 miles per hour. You'll now apply the brakes hard enough to gradually reduce your speed to 35 miles per hour, and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.

If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Control braking is possible only while enough air remains in the air tanks. The spring brakes will come on when the air pressure drops into the range of 20 to 45 PSI. A heavily-loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly-loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on. It is much safer to stop while there is enough air in the tanks to use the foot brakes.

Anytime you park, use the parking brake. Pull the parking brake control knob out to apply the parking brakes. Push it in to release them. The control will be yellow, diamond-shaped knob labeled parking brakes. On older vehicles, they may be a round blue knob or some other shape, including a lever that swings from side to side, or up and down. Don't use the parking brakes if the brakes are very hot or if the brakes are wet in freezing temperatures.

If they are used while they are hot, they can be damaged by the heat. If they're used in freezing temperatures when the brakes are wet, they can freeze, so the vehicle cannot move. Use wheel chocks to hold the vehicle, let hot brakes cool before using the parking brakes. If the brakes are wet, use the brakes lightly to heat and dry them. If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil, otherwise, the brakes could fail.

Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage.

Safety as a Service

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