Section 5: Air Brakes

This section is for drivers who drive or tow vehicles with air brakes

This section tells you about air brakes. If you drive or tow a commercial vehicle(s) equipped with air brakes, you will be tested on the information in this section. If you want to tow a trailer with air brakes, you must also read Section 6: Combination Vehicles.

Air brakes use compressed air to make the brakes work and must be well maintained and used correctly.

Air brake systems are three braking systems combined:

  • 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 brake system failure.

CDL air brake requirements. For CDL purposes, a vehicle's air brake system must meet the above definition and must contain the following which will be checked during the pre-trip inspection test:

  • Air gauges.
  • Low pressure warning device(s).

If the vehicle you use for your driving test does not have these components, your vehicle will not be considered as having an air brake system and you will have a "No Air Brakes" restriction on your CDL.

Note: A full service brake application must deliver to all brake chambers not less than 90 percent of the air reservoir pressure remaining with the brakes applied (CVC §26502).

The Air Brake System

An air brake system is a system that uses air as a way to transmit pressure from the driver's control to the service brake. It also includes an air-overhydraulic brake system.

There are many parts to an air brake system. You should know about the parts discussed here.

Air Compressor and Governor

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is driven by 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" (fully charged) level (no higher than 130 pounds per square inch, or "psi"), the governor stops the compressor from building air pressure. When the tank pressure falls to the "cut-in" pressure (no lower than 85 psi), the governor allows the compressor to start building pressure again.

Air Storage Tanks and Air Tanks Drains

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. This is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water, oil, etc., tends to collect in the bottom of the air tank. Each air tank is equipped with a drain valve in the bottom. There are three types:

  • Manually operated by turning a quarter turn, shown in Figure 5-1, or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
  • Automatic. The water and oil are automatically expelled from the valve (these valves are equipped for manual draining as well).
  • Air Dryer. The water and oil are automatically expelled from a spit valve. This type of valve "spits" out water and air each time the governor cycles.

The automatic types are available with electric heating devices. These help prevent freeze up of the automatic drain in cold weather.

a diagram of a reservoir

Alcohol Evaporator

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 storage tanks, valves, and other parts during cold weather. Ice inside the system can cause brake failure.

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).

Safety Valve

A safety relief 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 air pressure. The valve is usually set to open at 150 psi If the safety valve releases air, something is wrong with the brake system.

Brake Pedal

You apply the brakes by pushing down the brake pedal (also called the foot valve or treadle valve). The harder you push down on the pedal, the more air pressure is applied from the storage tanks into the brake chambers. Letting up on the brake pedal exhausts the air pressure from the brake chambers and releases the brakes. The air pressure used to apply the brakes must be built up in the reservoirs by the compressor. Pressing and releasing the pedal (fanning) can unnecessarily let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.

When you push the brake pedal down, two forces push back against your foot. One force comes from a spring in the valve. The second force comes from the air pressure going to the brake chambers. This lets you feel how much air pressure is being applied to the brake chambers. This "feel" does not tell you how much force is being applied to the brakes because that depends on brake adjustment.

Drum Brakes

Drum brakes (foundation brakes) may be used at each wheel. The most common type is the S-cam drum brake (so called because of its shape) shown in Figure 5-2.

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 cause brake failure.

S-cam brakes. When you push the brake pedal, air is let into each brake chamber (Figure 5-2). Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake cam shaft. This turns the S-cam. 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.

a diagram of a S-cam Air Brake

CamLaster. The CamLaster brake has two key design differences over traditional S-cam brakes. One feature is a completely internal adjustment system which is designed to continually keep the brake in proper adjustment. S-cam brakes, on the other hand, require an external slack adjuster. The second feature is a unique cam design that applies the brake shoe. Unlike a standard drum brake that has either a single or double anchor-pin brake, CamLaster slides the shoes down an inclined ramp on a cam to evenly contact the brake drum.

Wedge brakes. 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. 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 C-clamp.

One-Way Check Valve

This device allows air to flow in one direction only. All air tanks on air-braked vehicles must have a check valve located between the air compressor and the first reservoir (CVC §26507). The check valve keeps air from going out if the air compressor develops a leak.

Air Supply Pressure Gauge

All air-braked vehicles have an air supply 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, sometimes, a single gauge with two needles. Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.

Application Pressure Gauge

This gauge shows how much air pressure you are applying to the brakes (some vehicles do not have this gauge). When going down steep grades, increasing brake pressure to hold the same speed means the brakes are fading. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.

Low Air Pressure Warning Device

A low air pressure warning device is required on vehicles with air brakes. A warning device which you can see must come on when the air supply pressure drops between 55 to 75 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.

Another type of warning is the "wig wag." This device drops a mechanical arm into your view when the pressure in the system drops between 55 to 75 psi An automatic wig wag will rise out of your view when the pressure in the system goes above 55 or more psi. The manual reset type must be placed in the "out of view" position manually. It will not stay in place until the pressure in the system is between 55 to 75 psi.

On large buses, it is common for the low pressure warning devices to signal at 80 to 85 psi. Farm labor vehicles and Type I school buses must have both an audible and visual type warning device.

Stop Light Switch

Drivers behind you must be warned when you apply your brakes. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you apply the air brakes.

Front Brake Limiting Valve

Some 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 are used to reduce the chance of the front wheels skidding on slippery surfaces. However, they also 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 stepped on very hard (60 psi or more application pressure). These valves cannot be controlled by the driver. (Note: Some vehicles are manufactured with no front brakes.)

Spring Brakes

All trucks, truck tractors, and buses using air pressure to apply the service brakes must be equipped with emergency brakes and parking brakes. The parking brake must be held on by mechanical force (because air pressure can eventually leak away). Spring brakes are usually used to meet the emergency and parking brake requirements. 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 spring brakes. This lets the springs put on the brakes. A leak in the air brake system will generally 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, neither the regular brakes nor the emergency/parking brakes will work correctly.

Parking Brake Controls

In newer vehicles with air brakes, set the parking brakes using a diamond shaped, yellow, push-pull control knob. Pull the knob out to set the parking brakes (spring brakes), 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. If your vehicle is not equipped with an anti-compound system (only in vehicles with air brakes), you should not 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.

Modulating control valves. In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring loaded so you have a feel for the braking 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 the locking device.

Dual parking control valve. 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. 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.

Dual Air Brake Systems

Most newer 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 system operates the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer, if there is one. The first system is called the primary system and the other 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). The low air pressure warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 55 psi.

The warning system devices should come on before the air pressure drops below 55 psi in either system. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brake system fixed.

Antilock Braking Systems (ABS)

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 ABS. 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. Some ABS information:

  • Vehicles with ABS have yellow malfunction lamps to tell you if something isn't working.
  • Tractors, trucks, and buses have yellow ABS malfunction lamps on the instrument panel when you turn on the ignition.
  • Trailers will have yellow ABS malfunction lamps on either the left side, front or rear corner.
  • Dollies manufactured on or after March 1, 1998, are required to have a malfunction lamp on the left side.

On newer vehicles, the malfunction lamp comes on at start-up for a bulb check, and then goes out quickly. On older systems, the lamp could stay on until you are driving over five mph.

  • If the lamp stays on after the bulb check, or goes on while you are driving, you may have lost ABS control at one or more wheels.
  • In the case of towed units manufactured before ABS were 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 and changes the way you brake in an emergency.
  • ABS does not shorten your stopping distance, it helps keep the vehicle under control during hard braking.

Combination Vehicle Air Brakes

The trailer hand valve (also called the trolley valve or Johnson bar) works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not slow a tractor and semitrailer combination by using only the trailer hand brake. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all of the brakes on the vehicle including the trailers. There is much less danger of causing a skid or jackknife when using just the foot brake.

Trailer Hand Valve

Never use the hand valve for parking, because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes). Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.

Tractor Protection Valve

The tractor protection valve keeps air in the tractor or truck if the trailer breaks away or develops a bad leak. The tractor protection valve is controlled by the trailer air supply control valve in the cab. The control valve allows you to open and shut the tractor protection valve. It will close automatically if air pressure is low (in the range of 20 to 45 psi). When the valve closes, it stops any air from escaping and lets the air out of the trailer emergency line which causes the trailer emergency brakes to come on. (Emergency brakes are covered later.)

The trailer air supply control on newer vehicles is a red 8-sided knob which controls the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out and close the tractor protection valve when the air pressure drops into the range 20 to 45 psi. Emergency valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The normal position is used for pulling a trailer. The emergency position is used to shut the air off and put on the trailer emergency brakes.

Trailer Air Lines

Every combination vehicle has two air lines—the service line and the emergency line. They run between each vehicle (tractor to trailer, trailer to dolly, dolly to second trailer, etc.).

Service air line (normally blue). The service line (also called the control line or signal line) carries air which is controlled by the foot brake or the trailer hand brake. The pressure in the service line will similarly change depending on how hard you press the foot brake or hand valve. The service line is connected to a relay valve on the trailer to apply more or less pressure to the trailer brakes. The relay valve connects the trailer air tanks to the trailer air brakes. As pressure builds up in the service line, the relay valve opens and sends air pressure from the trailer air tank to the trailer brake chambers, putting on the trailer brakes.

Emergency air line (normally red). The emergency line has two purposes. First, it supplies air to the trailer air tanks and secondly, the emergency line controls the emergency brakes on combination vehicles. Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The pressure loss could be caused by a trailer breaking loose, tearing apart the emergency air hose. It could also be caused by a hose, metal tubing, or other part which breaks, letting the air out. When the emergency line loses pressure, it causes the tractor protection valve to close (the air supply knob will pop out).

Hose Couplers (Glad Hands)

Glad hands are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. The couplers have a rubber seal which prevents air from escaping. Clean the couplers and rubber seals before a connection is made. When connecting the glad hands, press the two seals together with the couplers at a 90° angle to each other. A turn of the glad hand attached to the hose will join and lock the couplers.

It is very important to keep the air supply clean. To keep the air supply clean, some vehicles have "dead end" or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air lines. Use the dummy couplers, if available, when the air lines are not connected to a trailer.

To avoid mistakes, metal tags are sometimes attached to the lines with the words service or emergency stamped on them. Sometimes colors are used. Blue is used for the service lines and red for the emergency lines.

If you do cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. Air will not be available to release the trailer spring brakes (parking brakes). If the spring brakes don't release when you push the trailer air supply control, check the air line connections.

Older trailers do not have spring brakes. If the air supply in the trailer air tank has leaked away there will be no emergency brakes and the trailer wheels will turn freely. If you crossed the air lines, you could drive away but you would not have trailer brakes. Before driving, always test the trailer brakes with the hand valve or by pulling the air supply control. Pull gently against them in a low gear to make sure the brakes work.

Trailer Air Tanks

Each trailer and converter dolly has one or more air tanks. They are filled by the emergency supply line from the tractor and they provide the air pressure used to operate trailer brakes. Air pressure is sent from the air tanks to the brakes by relay valves. The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The pressure in the service line is controlled by the brake pedal and the trailer hand brake.

It is important that you do not let water or oil build up in the air tanks. If you do, the brakes may not work. Each tank has a drain valve on it, and must be drained every day. If your tanks have automatic drains, they will keep most moisture out. However, you should still open the drains to check for moisture.

Shut-Off Valves

Shut-off valves (also called cut-out cocks) are used in the service and supply air lines at the back of the trailers used to tow other trailers. These valves permit closing the air lines when no other trailer is being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

Service, Spring, and Emergency Brakes

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. These have emergency brakes which work from the air stored in the trailer air tank. The emergency brakes come on whenever air pressure in the emergency line is lost. These trailers have no parking brake. The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected. The brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away and then there will be no brakes. It is very important that you use wheel chocks when you park trailers without spring brakes.

A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on.

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

Inspecting the Air Brake System

Use the basic inspection procedures described in Sections 2 or 4 to inspect your vehicle. There are more items to inspect on a vehicle with air brakes than one without them.

Engine Compartment

Check air compressor drive belt. If the air compressor is belt-driven, check for excessive wear, cracks, and tightness of the belt.


Check brake adjustment on S-cam brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Release the parking brakes on the truck or tractor and the emergency brakes on the trailer so you can mark the push rod in the unapplied position. Make a mark on the push rod with a chalk or scribe close to the brake chamber where the push rod comes out of the air chamber. Apply the truck or tractor parking brake and the trailer emergency braking system. Measure the travel of the push rod from the brake chamber and the mark you made with the chalk or scribe at each brake chamber. The push rod should move less than one inch on most brakes. (Smaller brake cams will have less push rod travel.)

If the brake push rod exceeds the required adjustment, adjust it or have it adjusted. (You are not expected to adjust them during the pre-trip test but you are expected to describe how to check that the brake push rod is adjusted properly.) Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the problem most often found in roadside inspections. Be safe—check the slack adjusters.

All vehicles built since 1994 have automatic slack adjustors. Even though automatic slack adjustors adjust themselves during full brake applications, they must be checked.

Automatic adjustors should not have to be manually adjusted, except when performing maintenance on the brakes and during installation of the slack adjusters. In a vehicle equipped with automatic slack adjusters, when the pushrod stroke exceeds the legal brake adjustment limit, it is an indication that there is a mechanical problem with the adjuster itself, or the related foundation brake components, or the adjuster was improperly installed.

The manual adjustment of an automatic adjuster to bring a brake pushrod stroke within legal limits is generally masking a mechanical problem, not fixing it. Additionally, manually adjusting most automatic adjusters will likely result in premature wear of the adjuster. When brakes equipped with automatic adjusters are found to be out of adjustment, the driver should take the vehicle to a repair facility as soon as possible to have the problem corrected.

The manual adjustment of an automatic adjuster should only be used as a temporary measure to correct the adjustment in an emergency situation as it is likely the brake will soon be back out of adjustment since this procedure usually does not fix the underlying adjustment problem.

(Note: Automatic slack adjusters made by different manufacturers do not all operate the same. Therefore, the specific Manufacturer's Service Manual should be consulted prior to troubleshooting a brake adjustment problem.)

Check brake drums (or discs), linings, and hoses. Brake drums must not have cracks. Linings must not be loose or soaked with oil or grease. They should not be thinner than the manufacturers specifications recommend. (Generally, this will be 1/4 inch.) Mechanical parts must be in place, not broken or missing. Check the air hoses connected to the brake chambers to make sure they are not cracked, cut, or worn.

In-Cab Air Brake Check

Note: All the Air Brakes system tests in this section are considered important and each can be considered critical parts of the in-cab air brakes tests. The items marked with an asterisk (*) in this section are required for testing purposes during the pre-trip portion of the CDL driving test. They may be performed in any order as long as they are performed correctly and effectively. If these items are not demonstrated and the parameters for each test are not verbalized correctly, it is considered an automatic failure of the pre-trip portion of the test.

Testing air leakage rate. There are two tests as follows:

Static Leakage Test

With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for one minute. The air pressure should not drop more than:

  • 2 psi for single vehicles.
  • 3 psi for a combination of two vehicles.
  • 5 psi for a combination of three or more vehicles.

An air loss greater than those shown indicate a problem in the braking system and repairs are needed before operating the vehicle.

*Applied Leakage Test

With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes so the entire system is charged. Allow the system to settle (air gauge needle stops moving), apply firm, steady pressure to the brake pedal (brake on), and hold.

After the system settles again, time for one minute. The air pressure should not drop more than:

  • 3 psi for single vehicles.
  • 4 psi for a combination of two vehicles.
  • 6 psi for a combination of three or more vehicles.

An air loss greater than those shown indicate a problem in the braking system and repairs are needed before operating the vehicle.

Note: You must be able to demonstrate this test and verbalize the allowable air loss for the examiner on this test.

If the air loss is too much, check for air leaks and fix. For testing purposes, identify if the air loss rate is too much.

*Air Compressor Governor Cut-Out Pressure Test

To perform this test, the air pressure for the vehicle must be rising when the engine is running. Run the engine at a fast idle. The air compressor governor must cut-out prior to the needle reaching 130 psi. Where the needle stops rising is the governor cut-out pressure.

For testing purposes, identify where the air governor cuts out the compressor and verbalize the maximum pressure at which this can occur.

Note: The air dryer exhausting should not be referenced as governor cut-out.

*Air Compressor Governor Cut-In Pressure Test

To perform this test, the air pressure for the vehicle cannot be rising when the engine is running. With the engine idling, slowly pump the brake pedal to reduce the air tank pressure. Watch the air pressure gauge between pumps to identify when the compressor cuts in (needle starts to rise). This should occur no lower than 85 psi.

For testing purposes, identify where the air governor cuts in the compressor and verbalize the minimum pressure at which this can occur.

*Low Air Pressure Warning Device Test

This test may be performed with engine on or off. To perform the test with the engine off, turn the electrical power on and have enough air pressure to keep the low air pressure warning device from coming on. Slowly pump the brake pedal to reduce air tank pressure. The low air pressure warning device must activate between 55 and 75 psi. For testing purposes, identify when the warning signal activates, and verbalize the legal range in which the signal must activate.

If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

*Check that the spring brakes come on automatically. Chock the wheels. Release all parking brakes and shut the engine off. Pump the brake pedal to reduce the air tank pressure. The trailer air supply valve knob should pop out when the air pressure falls to the manufacturer's specifications (usually in a range between 20 to 45 psi). This causes the spring brakes to engage. Some trailers use an air applied emergency brake system and some trailers use spring brakes as the emergency brake system.

Check rate of air pressure buildup. With the engine at operating rpms, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer's specifications. In single air systems (pre-1975), typical requirements are pressure buildup from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 rpms.

If air pressure does not build up fast enough, the pressure may drop too low during driving, requiring an emergency stop.

Test service brakes. Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph), and apply the brakes firmly using the brake pedal. Any pulling to one side, unusual feel, or delayed stopping action should be checked.

Test parking brake. Fasten your seat belt. Set the parking brake and try to move the vehicle or allow the vehicle to slowly move forward and apply the parking brake. The parking brake should stop a rolling vehicle, or not allow any movement.

Combination Vehicle Brake Check

In addition to those already listed in Section 2, complete these checks.

Check that air flows to all trailers (double or triple trailers):

  • Use the tractor parking brake and/or chock the wheels to hold the vehicle.
  • Wait for air pressure to reach normal.
  • Use the trailer hand brake to provide air to the service line.
  • Open the emergency line shut-off valve at the rear of the last trailer (you should hear air escaping).
  • Close the emergency line valve.
  • Open the service line valve to check that service pressure goes through all the trailers, then close the valve.

If you do not hear air escaping from both lines, check that the shut-off valves on the other trailer(s) and dolly(ies) are in the OPEN position. You must have air all the way to the back for all the brakes to work.

Test tractor protection valve:

  • Charge the trailer air brake system. (That is, build up normal air pressure and push in the air supply knob.)
  • Shut off the engine.
  • Pump the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control should pop out or go from the normal to the emergency position when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

If the tractor protection valve does not work correctly, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control.

Test trailer emergency brakes:

  • Charge the trailer air brake system and check that the trailer rolls freely.
  • Stop and pull out the trailer air supply control valve or place it in the emergency position.
  • Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.

Test trailer service brakes:

  • Check for normal air pressure.
  • Release parking brakes.
  • Move the vehicle forward, slowly.
  • Apply the trailer brakes with the hand control (trolley valve), if so equipped.

You should feel the brakes come on. This tells you the trailer brakes are connected and working. The trailer brakes should be tested with the hand valve but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.

Using Air Brakes

Push the brake pedal down until the vehicle comes to a smooth stop. If you have a manual transmission, don't push the clutch in until the engine rpm is close to idle. When you are stopped, select a starting gear. You should brake so you can steer and keep your vehicle in a straight line and allow you to turn if it becomes necessary. Use one of the following methods.

Braking with Antilock Brakes
  • When you brake hard on slippery surfaces in a vehicle without ABS, your wheels may lock up. When your steering wheels lock up, you lose steering control. When your other wheels lock up, you may skid, jackknife, or even spin the vehicle.
  • ABS helps you avoid wheel lock up. The computer senses impending lockup, reduces the braking pressure to a safe level, and you maintain control.
  • You may not be able to stop faster with ABS, but you should be able to steer around an obstacle while braking, and avoid skids caused by over-braking.
  • Having ABS on only the tractor, only the trailer, or even on only one axle, still gives you more control over the vehicle during braking. Brake normally.
  • When only the tractor has ABS, you should be able to maintain steering control, and there is less chance of jackknifing. But keep your eye on the trailer and let up on the brakes (if you can safely do so) if it begins to swing out.
  • When only the trailer has ABS, the trailer is less likely to swing out. But if you lose steering control or start a tractor jackknife, let up on the brakes (if you can safely do so) until you gain control.
  • When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:
    • Use only the braking force necessary to stop safely and stay in control.
    • Brake the same way, regardless of whether you have ABS on the tractor, the trailer, or both.
    • As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.
    • There is only one exception to this procedure; if you always drive a straight truck or combination with working ABS on all axles, in an emergency stop, you can fully apply the brakes.
    • Remember, if your ABS malfunctions, you still have regular brakes. Drive normally, but get the system serviced soon.
    • Without ABS, you still have normal brake functions. Drive and brake as you always have.
Emergency Stops

Controlled braking. This method is also called "squeeze" braking. Put on the brakes as hard as you can without locking the wheels. Do not turn the steering wheel while doing this. If you need to make large steering adjustments or if you feel the wheels sliding, release the brakes. Brake again as soon as the tires get traction.

Stab braking. (Only on vehicles without antilock brake systems.)

  • Press on the brake pedal as hard as you can.
  • Release the brakes when the wheels lock up.
  • As soon as the wheels start rolling, put on the brakes fully again.

It can take up to one second for the wheels to start rolling after you release the brakes. Make sure you stay off the brakes long enough to get the wheels rolling again. Otherwise the vehicle may not stay in a straight line.

Note: If you drive a vehicle with antilock brakes, you should read and follow the directions found in the owner's manual for stopping quickly.

Stopping Distance

This was discussed in Section 2 under Speed and Stopping Distances. With air brakes there is an added delay: the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light/medium trucks), the brakes work instantly. However, with air brakes, it takes time (up to one half second) 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
+ Effective braking distance

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. So at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over 300 feet. This is longer than a football field. Brakes get hot from use and will stop working if there is too much heat. Excessive heat is caused by trying to slow down too many times or too quickly from a high speed. Brakes will fade when they get too hot and will not slow you.

Brake Fade or Failure

Brakes are designed so brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates 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 braking effect of the engine.

Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat which causes chemical changes in the brake lining and expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move farther to contact the drums, and the force of this contact is also reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed or stopped at all.

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 sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.

Proper Braking Technique

Remember: The use of brakes on a long and/ or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

  • Apply the brakes just hard enough to feel a definite slowdown.
  • When your speed has been reduced to approximately 5 mph below your "safe" speed, release the brakes. (This brake application should last for about three seconds.)
  • When your speed has increased to your "safe" speed, repeat the steps above.

For example, if your "safe" speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.

Low Air Pressure Warning Signal

If a low air pressure warning signal comes on, stop and safely park your vehicle as soon as possible. There may be an air leak in the system(s). Controlled 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 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. Controlled braking is possible only while enough air remains in the air tanks.

Parking Brakes

Any time you park, use the parking brakes, except:

  • If the brakes are very hot, they can be damaged by the heat.
  • In freezing temperatures, if the brakes are very wet, they will freeze and the vehicle will be immobilized.
Air Tank Drains

Pull the parking brake control knob out to apply the parking brakes, push it in to release them. The control will be a yellow, diamond-shaped knob labeled "parking brakes" on newer vehicles. On older vehicles, it may be a round blue knob or some other shape (including a lever that swings from side to side or up and down).

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 while driving in a low gear 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.

Section 4: Transporting Passengers Safely | Table of Contents | Section 6: Combination Vehicles