Lesson # 17


This chapter covers the air brake systems used on trucks, buses, and trailers. Air brakes uses compressed air to make them work. You can apply all the braking force you need to each wheel of a heavy vehicle, even when pulling a trailer. Air brakes, if maintained and used properly, will safely stop large vehicles. You must know more about air brakes than the simpler brake systems used on light vehicles. Therefore, study this chapter closely.

Air brake systems are three braking systems combined. They are described as follows:


The following describes each part of the air brake system and its function.


When using the manual or automatic drain valves, make sure no oil leaks and contaminate the environment. Clean up any oil spill IAW unit environment SOP.

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This reduces the risk of ice in air brake valves and other brake parts during cold weather. Ice inside the system can make the brakes inoperative. Check the alcohol cent airier and till every day during cold weather, if necessary. You must still drain the air tank daily to get rid of water and oil (unless the drain valves are automatic).

A safety relief valve is installed in the first tank to which the air compressor pumps air. The safety valve protects the tank and the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, the system is defective. Contact your maintenance section and have the safety valve fixed before you operate the vehicle.

Push down on the brake pedal (also called the foot valve or treadle valve) to apply the brakes. Push the pedal down harder to apply more air pressure. Let up on the brake pedal to reduce the air pressure and release the brakes. Releasing the brakes lets some compressed air out of the system. This reduces the air pressure in the tanks. The air compressor must replace the expended air. If you press and release the pedal unnecessarily, you let out air faster than the compressor can replace it. If the pressure gets too low, the brakes will fail. When you push the brake pedal down, two forces push against your foot: one force comes from a spring; and the second, from the air pressure going to the brakes.

Foundation Brakes. Each wheel uses foundation brakes. 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 that slows the vehicle and creates heat. The heat a drum can take without damage depends on how hard and how long you use the brakes. Too much heat can stop them from working. The following describes different types of brakes used on vehicles.


Wedge brakes and disc brakes are less common than S-cam brakes.

All air-braked vehicles have supply pressure gauges connected to the air tank. These gauges tell you how much pressure is in the air tanks. If the vehicle has a dual system, each system has a gauge, or there is a single gauge with two needles.

The application pressure gauge shows how much air pressure you are applying to the brakes. This gauge is not installed on all vehicles. If you need to use increased pressure to hold the same speed going down grades, the brakes are fading. Slow down and use a lower gear. Brakes out of adjustment, air leaks, or mechanical problems can require you to increase pressure.

Vehicles with air brakes must have a low air pressure warning signal. It must activate before the air pressure in the tanks falls below 60 psi. On older model vehicles, it is half the compressor governor cut-out pressure. For example, on M52 tractors, the governor cut-out pressure is 90 psi. The warning buzzer should activate at approximately 45 psi. The warning is usually a red light or a buzzer. Another type of warning is the wig wag. This device drops a mechanical arm into view when the air pressure drops below 60 psi. An automatic wig wag will rise out of view when the air pressure goes above 60 psi. You must place the manual reset type in the out-of-view position. It will not stay in place until the pressure in the system is above 60 psi. On large buses, low pressure warning devices commonly signal at 80 to 85 psi.

You must warn drivers behind you when you apply the 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.

Some older vehicles (before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked normal and slippery. When the control is 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 reduce the vehicle’s stopping power. Front-wheel braking is good under all conditions. Tests have shown that front-wheel skids from braking are not likely even on ice. For normal stopping power, ensure the control is in the normal position. Many vehicles have automatic front-wheel limiting valves. They reduce the air to the front brakes, unless you apply the brakes very hard (60 psi or more application pressure). Drivers cannot control these valves.

All straight trucks, truck tractors, and buses must have emergency and parking brakes. Mechanical force must hold them on because air pressure can eventually leak away, Spring brakes are usually used to meet these needs. When driving, air pressure holds back powerful spring.. If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab lets you release the air from the spring brakes. Then the springs activate the brakes. A leak in the air brake system that causes all the air to be lost will cause the springs to activate the brakes. Straight truck and truck tractor spring brakes will fully activate when air pressure drops to 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, safely stop the vehicle while you can still control the brakes. The braking power of spring brakes depends on the adjustment of the brakes. If the brakes are not properly adjusted, neither the regular brakes nor the emergency parking brakes will work right.

In newer vehicles with air brakes, a yellow, diamond-shaped control knob puts on the parking brakes. Pull the knob out to engage the parking brakes (spring brakes). Push in to release. On older vehicles, a lever may control the parking brakes. Use the parking brakes whenever you park. The following describes how to apply parking brakes.


Never push the brake pedal down when the spring brakes are on. The combined forces of springs and air pressure could damage the brakes. 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.

Some vehicles have a control handle on the dash board to apply spring brakes gradually. This modulating valve is spring loaded so you can feel the braking action. The more the control lever is moved, the harder the braking. 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. Hold it in place with the locking device.

Some vehicles have dual parking control valves. When main air pressure is lost, the spring brakes activate. Some vehicles, such as buses, have a separate air tank that can be used to release the spring brakes. This lets you move the vehicle in an emergency. One of the valves is a push-pull type 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 are reactivated. The separate tank only has enough air to do this a few times. So, plan carefully when moving. Otherwise, you maybe stopped in a dangerous location when the separate air supply is exhausted.

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 with a single set of brake controls. Each system has its own air tanks, hoses, and lines. One system operates the regular brakes on the rear axles; the other, 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 the primary system; the other, the secondary system.

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum pressure of 100 psi in the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has two needles in one gauge). Check the low air pressure warning light and buzzer. The warning light and buzzer should shut off when the air pressure in both systems rises to a value set by the manufacturer. This value must be 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, stop right away and safely park the vehicle, If one air system is very low on pressure, either the front or rear brakes will not be operating fully. (This means you will take longer to stop.) Bring the vehicle to a safe stop. Get the air brake system fixed.

AIR BRAKE SYSTEM INSPECTION. The main reason for a proper PMCS is safety.  One of the most important components of a safe vehicle is a sound braking system.  Use the basic seven-step inspection procedure to inspect your vehicle.

Step 1- Check the Vehicle’s General Condition.

Step 2- Check the Engine Compartment.

Step 3- Get in the Vehicle and Check the Gages and Controls.

Step 4- Turn Off the Engine and Check the Lights.

Step 5- Do a Walkaround Inspection.

Step 6- Check Signal Lights.

Step 7- Check the Brake System.

A vehicle with air brakes has more things to inspect than one without them.

AIR BRAKE USE. The following describes how to use the air brake in different situations.

Normal Stops. For normal stops, push the brake pedal down. Control the pressure so the vehicle comes to a smooth and safe stop. If you have a manual transmission, do not push the clutch in until the engine RPM is close to idle speed.

Emergency Stops. For emergency stops, brake so you can steer and control the vehicle to stay in a straight line. Use one of the following two methods:


The M939 series trucks carries with it a known risk. They have conventional air brake systems. These air brakes are very sensitive. Air brakes are unique in that braking force is proportional to pedal travel, but the driver does not experience resistance from the brake pedal. The driver undergoing training may respond to this lack of resistance by applying too much force to the brake pedal. This causes the brakes to lockup and the vehicle to become uncontrollable. Drivers of these trucks must be well trained in operating tactical trucks with air brakes.


When driving the M939 series truck, apply brakes gradually when stopping. Panic stops will cause vehicle wheels to lock and the engine to stall. Power steering will be lost. Failure to apply brakes gradually may result in injury or death.


If you drive a vehicle with anti-lock brakes, you should read and follow the directions found in the Owner’s Manual for stopping quickly.

STOPPING DISTANCE. Driving too fast is a major cause of fatal crashes. You must adjust your speed to the driving conditions. These include traction, curves, visibility, traffic, and hills. Speed affects stopping distance. Whenever you double your speed, you need about four times as much distance to stop. Your vehicle will have four times the destructive power if it crashes. By slowing down, it reduces braking distance. Normal brakes are affected by three factors; Perception Distance, Reaction Distance, and Braking Distance.

Air brakes have an added delay: the time required for the brakes to work after you push the brake pedal. With hydraulic brakes (used on cars and Iight/medium trucks), the brakes work instantly. With air brakes, it takes time (1/2 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 has four different factors:


Perception Distance


  Reaction Distance


  Brake Lag Distance


  Effective Braking Distance


Total Stopping Distance

Total Stopping Distance. Perception distance plus reaction distance plus braking distance equals the total stopping distance. At 55 mph, it takes about 6 seconds to stop. The total distance your vehicle will travel is about the length of a football field: 60+60+ 170= 290 feet.

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. So at 55 mph, an average driver under good traction and brake conditions has a total stopping distance over 300 feet (longer than a football field).

When you use the brakes, 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. They can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.

Brakes, tires, springs, and shock absorbers on heavy vehicles work best when the vehicle is fully loaded. Empty trucks require greater stopping distances because an empty vehicle has less traction. The wheels of an empty vehicle tend to bounce and cause lockup of its wheels, thus creating poorer braking action (this is not usually the case with buses).

Excessive use of service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining that reduces friction and causes 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 reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed or stopped.

Brake fade is affected by adjustment. To safely control a vehicle, each brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are correctly adjusted. The correctly adjusted brakes can overheat and fade leaving you insufficient braking available to control the vehicle. Brakes can get out of adjustment quickly, especially when they are hot. Check them frequently for proper adjustment.

Use brakes as a supplement to the braking effect of the engine on long and/or steep downgrades, Once the vehicle is in the proper low gear, perform the following steps in proper braking techniques:

When your speed has again increased to your safe speed, repeat steps 1 and 2 above. For example: If your safe speed is 40 mph, do not apply the brakes until your speed reaches 40 mph. Now apply the brakes hard enough to gradually reduce your vehicle speed to 35 mph and then release the brakes. Repeat this as often as necessary until you reach the end of the downgrade.

If the low air pressure warning activates, stop and safely park the vehicle when possible. The system may have an air leak. Controlled braking is possible only while enough air remains in the air tanks. The spring brakes will come on when the air pressure drops to 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 you apply spring brakes. It is much safer to stop while the tanks have sufficient air to use the foot brake.

Any time you park your vehicle use the parking brakes, except as noted below. 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 another shape (including a lever that swings from side to side or up and down).

Do not use the parking brakes if the brakes are very hot or very wet in freezing temperatures. If you use them while they are hot, the heat can damage them. If you use them in freezing temperature when they are wet, they can freeze so the vehicle cannot move. Use wheel chocks to hold the vehicle. Let hot brakes cool before you use 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 after each working day. Otherwise, the brakes could fail.


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

TRAILER AIR BRAKESA truck with a trailer is usually heavier, larger, and requires more driving skill than a truck by itself. Thus, drivers required to tow a trailer need more knowledge and skill than drivers of single vehicles. This additional skill and knowledge extends to the braking systems of the truck and the trailer.   Safety and control of your vehicle is of primary concern.

Control your speed whether fully loaded or empty. Empty, large combination vehicles take longer to stop than fully loaded ones. When lightly loaded, the very stiff suspension springs and strong brakes of a trailer give poor traction. The wheels lockup very easily. Your trailer can swing out and strike other vehicles or jackknife.

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is known as off-tracking or cheating. Off-tracking causes the path of a truck and trailer to be wider than the truck by itself. Longer vehicles will off-track more. The truck's rear wheels will off-track some. The trailer’s rear wheels will off-track even more. Steer the front-end wide enough around a corner so the rear end does not run over the curb, pedestrians, or other vehicles. However, keep your vehicle’s rear near the curb to keep other drivers from passing you on the right. If you cannot turn without entering another traffic lane, turn wide as you complete the turn rather than swinging wide to the left before starting the turn. This will keep other drivers from passing you on the right. If drivers pass on the right, you might crash into them when you turn.

The Brake System. Every truck and trailer has two air lines: the service line and the emergency line. They run between each vehicle (truck to trailer).

Service Line. The service line (also called the control or signal line) carries air. The foot brake or the trailer hand brake controls the air pressure in the service line. Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change. The service line is connected to relay valves on the trailers 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. This sends air pressure from the trailer air tank to the trailer brake chambers and activates the trailer brakes.

Emergency Line. The emergency line (also called supply line) has two purposes:

Loss of air pressure in the emergency line activates the trailer emergency brakes. A trailer breaking loose, tearing apart the emergency air hose could cause the pressure loss. Or, a hose, tubing, or other part could break and let the air out. When the emergency line loses pressure, it also closes the tractor protection valve (air supply knob automatically pops out). Emergency lines are often coded red (red hose, red couplers, or other parts) to keep them from being confused with the blue or yellow service line.

Hose couplers (also called glad hands) are coupling devices that 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 connecting them. When connecting the glad hands, press the two seals together with the couplers at a 90-degree angle to each other. A turn of the glad hand attached to the hose will join and lock the couplers in position.

Some vehicles have dead-end or dummy couplers to which the hoses maybe attached when they are not in use. This keeps water and dirt from entering the coupler and the air lines. Use the dummy couplers when the air lines are not connected to a trailer. If there are no dummy couplers, you can sometimes lock the glad hands together (depending on the couplings). You must keep the air supply clean.

When coupling, be sure to couple the proper glad hands together. Blue or yellow is used for the service lines and red for the emergency (supply) lines. Sometimes the lines have metal tags attached with service or emergency stamped on them. If you cross the air lines, supply air will be sent to the service line instead of charging the trailer air tanks. Air will not be available to release the trailer spring brakes (parking brakes). If the spring brakes do not 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, you will not have emergency brakes. The trailer wheels will turn freely. If you crossed the air lines, you could drive but would not have trailer brakes. Always test the trailer brakes before driving. Use the hand valve or pull the air supply (tractor protection valve) control. Pull gently against them in a low gear to be sure the brakes work.

Each trailer and converter dolly has one or more air tanks. The emergency (supply) line from the tractor fills them. They provide the air pressure used to operate trailer brakes. Relay valves send air pressure from the air tanks to the brakes. The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The brake pedal (and the trailer hand brake) controls pressure in the service line. Do not let water and oil buildup in the air tanks or the brakes may not work properly. There is a drain valve on each tank. Drain each tank daily. If your tanks have automatic drains, they will keep most moisture out. However, you should still open the drains daily to be sure no moisture has collected.

Shutoff valves (also called cutout cocks) are used in the service and supply airlines at the back of trailers when towing other trailers. These valves permit the airlines to be closed when other trailers are not being towed. Be sure all shutoff valves are in the open position except the ones at the back of the last trailer; these must be closed.

Newer trailers have spring brakes just like trucks and truck tractors. However, trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes 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; then there will be no brakes. Therefore, for safety reasons, you must use wheel chocks when you park trailers without spring brakes. A major leak in the emergency line will close the tractor protection valve and activate the trailer emergency brakes. You may not notice a major leak in the service line until you try to apply the brakes. Air loss from the leak will lower the air tank pressure quickly. Low enough pressure will activate the trailer emergency brakes.

INSPECTING COMBINATION VEHICLES. A truck and trailer combination has more things to inspect than a single vehicle. Check the following during a walkaround inspection of a truck and trailer combination in addition to the items described earlier in this lesson for the truck alone:

Check the coupling system areas.

Check the truck and trailer combination brakes.

NOTE: If the tractor protection valve does not work right, 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 the air buzzer and air indicator light. With engine at idle speed (750-850 rpm), listen for air buzzer and watch air indicator light. The vehicle is not mission capable if the air buzzer and/or indicator light will not come on below 65 psi (448 kPa), or will not shut off above 65 psi (448 kPa). If air pressure reading is below 60 psi (414 kPa), braking will require excessive pedal force. Shut down engine and check to see what is wrong. Failure to obtain correct air pressure may result in injury or death to personnel.

Check the air pressure gauge.   The air pressure gage normal operating range is 90-120 psi. The vehicle is not mission capable if the air pressure gauge reads 60 psi or below.

Operate service brakes to determine stopping ability. Check for pulling to one side, grabbing, or other abnormal operation. The vehicle is not mission capable if the service brakes do not operate properly, or brake pedal goes within two inches of floorboard.

Check the parking brake. Engage the parking brake lever and place transmission select lever in DRIVE. Slowly raise engine RPM to 2,000 (for no more than 30 seconds). If vehicle moves, stop engine and adjust parking brake, as required, by turning knob on top of parking brake lever clockwise to increase braking action; counterclockwise to decrease braking action.  The vehicle is not mission capable if the vehicle continues to move with parking brake applied after adjustment.

Check air brake kit.  Check operation of airbrake (trailer) kit, if installed, and mission requires pulling load. Apply airbrakes on trailer and slowly pull trailer.  Check operation of trailer stoplights.  The trailer is not mission capable if the trailer can be moved after brakes are applied.

Check the brake pedal free travel. Brake pedal free travel should be between .25 and .50 inches.  If the free play exceeds .50 inches you should notify your maintenance supervisor.

Check the master cylinder reservoir fluid level.  Check the master cylinder reservoir fluid level.  If it is not at the full line add fluid as needed.  The 2-1/2 ton truck uses silicone brake fluid.