Why the air brakes test matters
Air brakes use compressed air to make the brakes work. They are safe when used and maintained correctly, but they work very differently from car brakes. If you fail the air brakes knowledge test, or take your skills test in a vehicle without air brakes, your CDL will carry a restriction that keeps you out of any CMV with air brakes. In many states the test has 25 questions, and you need 80% (20 correct) to pass.
An air brake vehicle really has three brake systems:
- Service brakes slow and stop you when you press the brake pedal.
- Parking brakes hold the vehicle when you apply the parking brake control.
- Emergency brakes use parts of the service and parking systems to stop you if the system fails.
Parts of an air brake system
Air flows from the compressor to the tanks, then to the brakes at each wheel when you press the pedal.
- Air compressor, driven by the engine.
- Governor, which tells the compressor when to stop and start pumping.
- Air storage tanks.
- Drain valve at the bottom of each tank.
- Brake pedal (foot valve), which sends air to the brakes.
- Brake chamber, which pushes a rod when air arrives.
- Slack adjuster, turned by the push rod.
- Brake drum, where the shoes press to slow the wheel.
| Part | What it does |
|---|---|
| Air compressor | Pumps air into the tanks. It is driven by the engine through gears or a V-belt. If it has its own oil supply, check the oil before driving. |
| Governor | Stops the compressor at about 125 psi (cut-out) and starts it again at about 100 psi (cut-in). |
| Air tanks | Hold enough air to use the brakes several times even if the compressor stops. |
| Tank drains | Let out water and oil that collect in the tanks. Manual drains must be drained at the end of each driving day. |
| Alcohol evaporator | Adds alcohol to cut ice in cold weather. Check and fill it daily in cold weather. You still have to drain the tanks. |
| Safety valve | On the first tank the compressor fills. It usually opens at 150 psi to protect the system. |
| Brake pedal (foot valve) | Pressing harder applies more air pressure. Releasing it lets air out. |
| Foundation brakes | The brakes at each wheel. S-cam drum brakes are the most common; wedge and disc brakes are less common. |
| Slack adjuster | Connects the brake chamber push rod to the camshaft, which turns the S-cam. |
| Supply pressure gauge | Shows how much air is in the tanks. |
| Application pressure gauge | Shows how much air pressure you are sending to the brakes (not on all vehicles). |
| Low air pressure warning | A red light, buzzer or wig wag that must come on before pressure drops below 60 psi. |
| Stop light switch | Turns on the brake lights when you apply the air brakes. |
In an S-cam brake, air pushes the brake chamber's push rod, which turns the slack adjuster and the camshaft. The S-shaped cam forces the brake shoes against the drum. When you let go of the pedal, a return spring pulls the shoes away. Too much heat in the drums can make the brakes fail.
Do not pump the brake pedal when you don't need to. Each time you let it up, air is let out, and the compressor may not keep up.
Spring brakes and the parking brake
Trucks, truck tractors and buses must have spring brakes. Powerful springs are held back by air pressure. If the air pressure drops to about 20 to 45 psi (usually 20 to 30), the springs put the brakes on. This is why you must stop safely as soon as the low air warning comes on.
On newer vehicles the parking brake control is a yellow, diamond-shaped knob. Pull it out to apply the parking brakes and push it in to release them. Never push the brake pedal down while the spring brakes are on; the combined force can damage the brakes.
Dual air brake systems
Many heavy vehicles have a dual air brake system: two separate air systems that share one set of controls. The primary system usually runs the rear brakes, and the secondary system runs the front brakes (and sometimes one rear axle). Before driving, let the compressor build at least 100 psi in both systems.
Antilock brakes (ABS)
Air-braked truck tractors built on or after March 1, 1997, and other air-braked vehicles built on or after March 1, 1998, must have ABS. A yellow lamp shows an ABS problem. ABS stops the wheels from locking so you can steer, but it does not always shorten your stopping distance. If ABS fails, your normal brakes still work.
Inspecting air brakes
Air brake checks are part of the seven-step inspection. In the engine compartment, check that a belt-driven compressor's belt is in good condition and tight. During the walkaround, park on level ground, chock the wheels and release the parking brakes so you can check the slack adjusters.
- Pull hard on each slack adjuster you can reach. If it moves more than about 1 inch where the push rod attaches, it probably needs adjustment.
- Brakes out of adjustment are the most common problem found in roadside inspections.
- Automatic slack adjusters (on vehicles built since 1994) must still be checked. Adjusting them by hand is only a temporary fix; have the vehicle repaired.
- Brake drum cracks must be no longer than half the width of the friction area.
- Linings must not be loose, soaked with oil or grease, or dangerously thin.
- Air hoses to the brake chambers must not be cut or worn.
The final air brake check (step 7)
| Check | How to do it | What you should see |
|---|---|---|
| Low pressure warning | Engine off, key on, pump the brake pedal to lower tank pressure | Warning comes on before pressure drops below 60 psi |
| Spring brakes | Keep pumping to lower pressure further | Parking brake knob and tractor protection valve pop out at the maker's pressure |
| Pressure build-up (dual system) | Engine at operating rpm | From 85 to 100 psi within about 45 seconds |
| Governor | Engine at fast idle, then lower pressure with the pedal | Cut-out near 125 psi, cut-in near 100 psi |
| Leakage, brakes released | Fully charged, engine off, parking brake released, time for 1 minute | Less than 2 psi (single vehicle) or 3 psi (combination) |
| Leakage, brakes applied | Hold the pedal at 90 psi or more for 1 minute after the first drop | No more than 3 psi (single) or 4 psi (combination) |
| Parking brake | Set it, then gently pull against it in a low gear | Vehicle does not move |
| Service brakes | Move forward at about 5 mph and brake firmly | No pulling to one side, no unusual feel or delay |
If pressure builds too slowly or the leakage is too high, do not drive until the system is fixed.
Using air brakes safely
Normal and emergency stops
For a normal stop, press the pedal smoothly. With a manual transmission, don't push in the clutch until the engine is down near idle. In an emergency, use one of these methods:
- Controlled braking: brake as hard as you can without locking the wheels. Keep steering movements small.
- Stab braking: apply the brakes fully, release when the wheels lock, and apply fully again as soon as they roll. It can take up to one second for the wheels to start rolling.
With working ABS on every axle, you may fully apply the brakes in an emergency stop. With ABS on only some axles, brake normally and ease off if the trailer starts to swing.
Stopping distance and brake lag
Air takes time to flow through the lines. This brake lag is about half a second or more, and at 55 mph it adds about 32 feet. Total stopping distance with air brakes is perception distance + reaction distance + brake lag distance + braking distance.
| Speed | Total stopping distance (dry road, air brakes) |
|---|---|
| 15 mph | About 46 feet |
| 30 mph | About 148 feet |
| 45 mph | About 310 feet |
| 50 mph | About 375 feet |
| 55 mph | About 451 feet |
Brake fade and downgrades
Using the brakes too much makes them very hot. The linings lose friction and the drums expand, so the brakes fade and may stop working. Brakes that are out of adjustment make the others work harder and fade sooner. If you must keep raising application pressure to hold your speed, your brakes are fading.
On a long downgrade, use a low gear and let the engine do most of the braking. Apply the brakes just hard enough to feel a definite slowdown. When you are about 5 mph below your safe speed, release them. Each application should last about 3 seconds. If your safe speed is 40 mph, brake to about 35 mph, release, and repeat.
Low air pressure
If the low air warning comes on, stop and park safely as soon as you can. You can only brake with control while there is enough air. When the spring brakes come on, a heavy vehicle will take longer to stop because spring brakes are not on every axle, and a light vehicle may skid.
Parking
- Use the parking brakes whenever you park, except as noted below.
- Do not set the parking brakes if the brakes are very hot; they can be damaged. Chock the wheels on level ground and let them cool.
- Do not set them if the brakes are very wet in freezing weather; they can freeze. Use them lightly in a low gear to dry them first.
- Never leave the vehicle unattended without setting the parking brakes or chocking the wheels.
- Drain tanks that have manual drains at the end of each working day.
Sources
- Commercial Driver License Manual, Section 5: Air Brakes (Rhode Island DMV)
- 49 CFR Part 383 Subpart F: Vehicle groups and endorsements (eCFR)
- 49 CFR Part 383 Subpart H: Tests and passing scores (eCFR)
- Washington State Department of Licensing: CDL knowledge test
- Utah Driver License Division: CDL written knowledge tests