Air Brakes for Commercial Drivers
Understand how a truck air brake system works, test it before you drive, and react correctly to low air pressure or brake fade.
Table of contents
Most heavy trucks and buses stop with compressed air instead of hydraulic fluid. Air brakes are powerful and reliable, but they behave differently from the brakes in a car. They take longer to respond, they depend on air you have to keep building, and they are designed to lock up on their own if the air is lost. A driver who understands why can catch a problem in the yard and stay in control on a long grade.
What You Will Be Able to Do
- Name the main parts of an air brake system and say what each one does.
- Explain how spring brakes hold a vehicle and why they apply when air is lost.
- Run the standard air brake checks and judge the results.
- Explain how a trailer gets its air and what protects the tractor if a line breaks.
- Respond correctly to a low air warning.
How the System Works
An air brake system has three working parts: the service brake system you use while driving, the parking brake system, and the emergency system that uses parts of both.
- An engine-driven compressor pumps air into storage tanks.
- The governor tells the compressor when to pump and when to stop.
- Air tanks hold the supply, enough to stop several times even if the compressor fails.
- Your foot on the foot valve, or treadle, sends air to each wheel.
- Air enters a brake chamber and pushes a rod.
- The rod moves a slack adjuster, a lever that turns a cam or works a disc caliper.
- The linings press on the drum or rotor, and the wheel slows.
Lift your foot and the air leaves through a valve. That is the hiss you hear. Every press uses air, so pressing and releasing again and again drains the supply faster than the compressor refills it.
Modern vehicles have a dual air system: two separate circuits with their own tanks and gauge needles. One usually serves the rear brakes and the other serves the front brakes. If one circuit fails, the other can still stop the vehicle, but not as well. Stop safely and do not drive on until it is repaired.
Parts You Should Know
| Part | Job |
|---|---|
| Governor | Stops the compressor at cut-out pressure, restarts it at cut-in |
| Safety relief valve | Releases air if pressure gets too high, usually set at 150 psi |
| Air dryer | Removes water and oil before they reach the tanks |
| Drain valves | Let you drain water and oil from the tanks |
| Low air warning | Buzzer or light that tells you pressure is falling |
| Stop light switch | Turns on the brake lights when you apply the brakes |
| Spring brakes | Strong springs held back by air, used for parking and emergencies |
| ABS | Keeps wheels from locking in a hard stop |
Water and oil collect in the tanks. Water can freeze in cold weather and cause brake failure. If your tanks have manual drains, drain them at the end of each day of driving. Do not rely on an automatic drain until you know how your vehicle is set up.
Spring Brakes
Service brakes need air to apply. Spring brakes are the opposite. A powerful spring wants to apply the brake, and air pressure holds the spring back.
- With full air, the spring is held back and the wheels roll free.
- When you pull the yellow diamond parking knob, the air is released and the springs apply the brakes.
- If the system loses air, the springs apply on their own, usually when pressure falls to somewhere between 20 and 45 psi.
This is the system's built-in safety net. It holds a parked vehicle without any air at all, and it stops a vehicle that has lost its air. It is not a good way to stop, though. The brakes come on hard and without warning, and you may have little control.
Warning: Do not use the parking brakes when the brakes are very hot from a steep grade, because they can be damaged, or when they are wet in freezing weather, because they can freeze. Use wheel chocks to hold the vehicle in those conditions.
The Numbers to Know
State CDL manuals use these figures. Your own state's manual is the final word for your test, and vehicle makers may give different values.
| Item | Typical value |
|---|---|
| Governor cut-in | About 100 psi |
| Governor cut-out | About 125 psi, and 120 to 140 psi on the skills test |
| Safety relief valve | About 150 psi |
| Pressure buildup | From 85 to 100 psi within 45 seconds |
| Low air warning | Before pressure falls below about 60 psi |
| Spring brakes apply | 20 to 45 psi |
| Static leak, brakes released | Under 2 psi a minute single, under 3 psi combination |
| Applied leak, pedal held | No more than 3 psi a minute single, 4 psi combination |
| Manual slack adjuster movement | No more than about 1 inch when pulled by hand |
The low air warning is the one figure you will see written two ways. The federal vehicle standard and most manuals use 60 psi. Some state test guides say 55 psi, which is the federal limit for older vehicles not built to that standard. Learn your state's number, and treat any warning at all as a reason to act.
Testing the System
Run these checks before you drive, in this order. Chock the wheels first.
1. Pressure Buildup
With the engine at operating speed, watch the gauge. Pressure should go from 85 to 100 psi within 45 seconds on a dual system. Vehicles with larger tanks can take longer and still be safe, so check the maker's figure. Slow buildup points to a leak or a worn compressor.
2. Governor Cut-Out and Cut-In
Let pressure build. The compressor should stop at about 120 to 140 psi, which you may notice as the air dryer spitting. Then pump the pedal to lower the pressure. The compressor should restart at about 100 psi.
3. Leak Rate
- Build to full pressure and shut off the engine.
- Release the parking brakes and let the system settle.
- Time one minute with your foot off the pedal. This is the static test.
- Press the pedal firmly and hold it for one minute. This is the applied test.
- Compare each loss with the limits in the table.
If the loss is over the limit, the vehicle stays parked until the leak is found and fixed.
4. Low Air Warning
Turn the key on with the engine off. Fan the brake pedal, meaning press and release it again and again, to lower the pressure. The warning must come on before about 60 psi. If it does not, you could lose air without knowing.
5. Spring Brakes Apply
Keep fanning. On a tractor or straight truck, the parking knob should pop out at about 20 to 45 psi. On a combination, the trailer supply knob should also pop out, which shows the tractor protection system is working.
6. Parking Brake Hold
Build pressure again, set the parking brake, and gently pull against it in a low gear. The vehicle should not move.
7. Service Brake Stop
Release the parking brake, move forward slowly at about 5 mph, and press the pedal firmly. The vehicle should stop straight with no pull to one side and no unusual delay.
Brake Adjustment
Brakes lose power as linings wear and rod travel grows. With the wheels chocked and the parking brake released, a manual slack adjuster should not move more than about 1 inch where the pushrod attaches. Many vehicles have automatic slack adjusters. They should not need manual adjustment in service, and a brake on one that is out of adjustment points to a problem for a mechanic to find.
Trailer Air
A trailer gets air through two lines connected by couplers called glad hands.
| Line | Color | Job |
|---|---|---|
| Service line | Blue | Carries the signal to apply the trailer's brakes when you press the pedal |
| Emergency or supply line | Red | Fills the trailer's tanks and holds off its spring brakes |
Crossing these lines means the trailer's brakes will not work as they should. Check the colors and test the brakes after every coupling.
The trailer air supply knob is the red eight-sided knob on newer vehicles. Push it in to send air to the trailer. Pull it out to shut the air off and apply the trailer's spring brakes.
The tractor protection valve closes when pressure falls into the 20 to 45 psi range, or when you pull the supply knob. If a trailer breaks away or a line tears off, this valve keeps the tractor's air from escaping, so you can still stop the tractor. When the emergency line loses pressure, the trailer's spring brakes apply.
Driving With Air Brakes
Brake lag. Air takes time to travel through the lines, about half a second. At 55 mph on dry pavement, that adds about 32 feet to your stopping distance, for a total of more than 450 feet. Leave more room than you think you need.
Low air on the road. If the warning comes on while you are driving:
- Slow down and stop as soon as you safely can, while you still have braking air.
- Pull well off the road and set the parking brake.
- Do not drive until the cause is fixed.
Long downgrades. Air brakes get their stopping power from friction, and friction makes heat. Overheated brakes fade, which means the drums expand away from the linings and the brakes lose power. Pick a low gear before you start down, and let the engine do most of the work. When you must use the brakes, use a firm application, not light steady pressure:
- Decide on your safe speed.
- When you reach it, press hard enough to feel a definite slowdown.
- When you are about 5 mph below your safe speed, release the brakes. Keep each application short, and do not ride the pedal.
- Let the speed come back up to your safe speed.
- Repeat.
Worked example: your safe speed for a grade is 40 mph. When the speedometer reaches 40, brake until it shows 35. Release, let it climb back to 40, and brake again.
Hard stops. With ABS, press firmly and hold. Do not pump. Without ABS, a controlled or stab braking technique keeps the wheels rolling so you can still steer.
Common Mistakes
| Mistake | What can happen |
|---|---|
| Fanning the brakes on the road | Pressure falls faster than the compressor can rebuild it |
| Light, steady braking on a long grade | Brakes overheat and fade |
| Driving before pressure is up | Spring brakes may not release fully, and you have less reserve |
| Skipping the tank drain | Water can freeze in lines and valves |
| Using the parking brake on hot brakes | Damaged drums and linings |
| Crossing the blue and red lines | Trailer brakes do not work as they should |
Check Your Understanding
You press and release the pedal repeatedly in stop-and-go traffic and see the pressure dropping. Why?
Every application uses air. Pressing and releasing again and again uses it faster than the compressor can replace it. Use steady, planned applications and leave more space.
During the applied leak test on a tractor-trailer, pressure falls 6 psi in one minute. Can you leave?
No. The limit for a combination is 4 psi a minute. Keep the vehicle parked and report the leak.
A trailer's red line tears off on the highway. What happens?
The trailer's emergency line loses pressure and its spring brakes apply. The tractor protection valve closes to save the tractor's air. Keep control and stop as safely as you can.
You stop on a cold, wet night after driving through slush. How should you hold the vehicle?
Wet brakes can freeze when the parking brakes are set. Use wheel chocks if it is safe and allowed by your carrier, and let the brakes dry first if you can.
Key Takeaways
- Air takes time to reach the wheels. Brake lag adds about 32 feet at 55 mph.
- Spring brakes use spring force, and air holds them off. Lose the air and they apply.
- Test buildup, governor, leak rate, low air warning and spring brakes before you drive.
- If the low air warning comes on, stop safely while you still have air.
- On a downgrade, choose a low gear first, then brake firmly and release instead of riding the pedal.