CDL test guide · 50 states
CDL Air Brakes Test Guide
Needed to drive a truck or bus with air brakes. Skip it and the CDL carries an air brake restriction. Taken for Class A, B or C.
What every manual says
From the parts of an air brake system to checking it before a trip and using it on the road.
Air Brakes
Air brakes are three braking systems: the service brake (applied and released with the brake pedal in normal driving), the parking brake (applied with the parking brake control) and the emergency brake (using parts of the service and parking systems to stop the vehicle if the brake system fails).
Air Compressor
The air compressor pumps air into the storage tanks; it is driven by the engine through gears or a v-belt, may be air-cooled or cooled by the engine cooling system, and may have its own oil supply (check its level before driving) or use engine oil.
Air Storage Tanks
Air storage tanks hold compressed air; their number and size vary, and they hold enough air to use the brakes several times even if the compressor stops working.
Air Tank Drains
Compressed air usually contains water and compressor oil, which are bad for the system (water can freeze and cause brake failure) and collect at the bottom of the tanks.
Alcohol Evaporator
Some systems have an alcohol evaporator that puts alcohol into the air to reduce the risk of ice in valves and other parts in cold weather (ice can stop the brakes working); check and fill it every day in cold weather, and still drain the tanks daily unless they drain automatically.
Safety Valve
A safety relief valve in the first tank the compressor fills protects the system from too much pressure; it usually opens at 150 psi, and if it releases air something is wrong and a mechanic should fix it.
The Brake Pedal
The brake pedal (foot or treadle valve) applies more air pressure the harder you push.
Foundation Brakes
In s-cam brakes, pressing the pedal lets air into each brake chamber; the pushrod moves the slack adjuster, twisting the brake camshaft and turning the S-shaped cam, which forces the shoes apart against the drum; on release the cam turns back and a spring pulls the shoes away.
Foundation Brakes
In wedge brakes, the pushrod drives a wedge between the ends of two shoes, pushing them against the drum (one or two chambers; self-adjusting or manual).
Supply Pressure Gauges
All air brake vehicles have a supply pressure gauge connected to the air tank showing the tank pressure; a dual system has a gauge for each half (or one gauge with two needles).
Stop Light Switch
An electric stop light switch worked by air pressure turns on the brake lights when you apply the air brakes, warning drivers behind you.
Spring Brakes
All trucks, truck tractors and buses must have emergency and parking brakes held on by mechanical force, since air can leak away.
Spring Brakes
Tractor and straight truck spring brakes come fully on when air pressure drops to 20–45 psi (typically 20–30 psi).
Parking Brake Controls
On newer vehicles the parking brakes are applied with a yellow, diamond-shaped push-pull knob: pull it out to apply, push it in to release (older vehicles may use a lever).
Parking Brake Controls
A modulating control valve is a spring-loaded dash handle that applies the spring brakes gradually (the more you move it, the harder they apply) so you can control them if the service brakes fail; to park, move it all the way and lock it.
Parking Brake Controls
Some vehicles, such as buses, have dual parking control valves and a separate air tank that can release the spring brakes after main air pressure is lost, so you can move in an emergency: one push-pull valve sets the parking brakes, and a spring-loaded one releases them while held in.
Dual Air Brake
Most heavy-duty vehicles use dual air brake systems: two separate systems, each with its own tanks, hoses and lines, sharing one set of brake controls. The primary system usually works the rear axle brakes and the secondary the front (and possibly one rear) axle; both supply a trailer.
Dual Air Brake
Before driving with a dual air system, let the compressor build at least 100 psi in both the primary and secondary systems.
Step 7 Final Air Brake Check
To test the low pressure warning: shut off the engine with enough pressure that the warning is off, turn on the electrical power and step on and off the brake to lower tank pressure.
Brake Fading or Failure
Overusing the service brakes causes overheating and brake fade: too much heat causes chemical changes in the lining that reduce friction, and expands the drums, so the shoes must move farther and press with less force.
Proper Braking Technique
On a long or steep downgrade the brakes only supplement engine braking: in the proper low gear, apply them just hard enough to feel a definite slowdown, release at about 5 mph below your safe speed (about three seconds), and repeat when you reach the safe speed again.
Low Air Pressure
If the low air warning comes on, stop and park safely as soon as possible, since there may be a leak and controlled braking is possible only while air remains.
Parking Brakes
Use the parking brakes any time you park, except when the brakes are very hot (they can be damaged) or very wet in freezing temperatures (they can freeze so the vehicle can’t move).
Every state's manual: Air Brakes
Each links its air brakes practice test.
- AlabamaSection 5 · p. 75–84
- AlaskaSection 5 · p. 78–87
- ArizonaSection 5 · p. 77–86
- ArkansasSection 5 · p. 72–81
- CaliforniaSection 5 · p. 101–113
- ColoradoSection 5 · p. 62–70
- ConnecticutSection 5 · p. 73–82
- DelawareSection 5 · p. 79–88
- FloridaSection 5 · p. 79–88
- GeorgiaSection 5 · p. 136–153
- HawaiiSection 5 · p. 81–90
- IdahoSection 5 · p. 74–83
- IllinoisSection 5 · p. 71–80
- IndianaSection 5 · p. 73–82
- IowaSection 5 · p. 77–86
- KansasSection 5 · p. 72–81
- KentuckySection 5 · p. 82–91
- LouisianaSection 5 · p. 92–101
- MaineSection 5 · p. 75–84
- MarylandSection 5 · p. 145–171
- MassachusettsSection 5 · p. 98–107
- MichiganSection 5 · p. 85–93
- MinnesotaSection 5 · p. 119–128
- MississippiSection 5 · p. 72–81
- MissouriSection 5 · p. 69–78
- MontanaSection 5 · p. 80–89
- NebraskaSection 5 · p. 84–93
- NevadaSection 5 · p. 80–89
- New HampshireSection 5 · p. 72–81
- New JerseySection 5 · p. 72–81
- New MexicoSection 5 · p. 75–84
- New YorkSection 5 · p. 87–100
- North CarolinaSection 5 · p. 77–86
- North DakotaSection 5 · p. 77–86
- OhioSection 5 · p. 69–78
- OklahomaSection 5 · p. 72–81
- OregonSection 5 · p. 75–83
- PennsylvaniaSection 5 · p. 95–104
- Rhode IslandSection 5 · p. 70–79
- South CarolinaSection 5 · p. 87–96
- South DakotaSection 5 · p. 95–104
- TennesseeSection 5 · p. 77–86
- TexasSection 5 · p. 81–90
- UtahSection 5 · p. 77–86
- VermontSection 5 · p. 85–94
- VirginiaSection 5 · p. 72–81
- WashingtonSection 5 · p. 79–88
- West VirginiaSection 5 · p. 110–119
- WisconsinSection 5 · p. 107–119
- WyomingSection 5 · p. 71–80