CDL Air Brakes · CDL Air Brakes Test
CDL Air Brakes: Parts of an Air Brake System Concepts
Importance ★★★★★ · 35% (estimate)
Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.
Overview
An air brake system is really three braking systems working together: the service brakes you use with the pedal, the parking brakes you use when you park, and the emergency brakes that stop the vehicle if the service brakes fail. Everything starts at the air compressor, which the engine drives to pump air into the storage tanks. The governor decides when the compressor pumps: it stops pumping at the cut-out pressure and starts again at the cut-in pressure. A safety relief valve on the first tank lets air out if pressure climbs too high; if it opens, something is wrong and should be fixed.
Compressed air carries water and compressor oil into the tanks, and in cold weather that water can freeze and make the brakes fail. That is why every tank has a drain valve. A manual drain is opened with a quarter turn of a drain cock or by pulling a cable, and you drain the tanks completely at the end of each day of driving. Some systems also have an alcohol evaporator that puts alcohol into the air to help keep ice out of the valves; check and fill it every day in cold weather.
At each wheel, the most common foundation brake is the S-cam drum brake. Air enters the brake chamber and pushes out a push rod, which moves the slack adjuster, which twists the camshaft and turns the S-cam. The S-cam spreads the brake shoes against the drum, and a return spring pulls them back when you let off the brakes. Air disc brakes use a power screw in place of the S-cam. Whatever the type, the brakes must stay in adjustment, and an automatic slack adjuster that keeps going out of adjustment points to a mechanical problem.
Spring brakes are held off by air pressure, so when air is lost, strong springs put the brakes on. On newer vehicles you control them with a yellow, diamond-shaped knob: pull it out to park, push it in to release. Gauges and warnings tell you what is happening: the supply gauge shows tank pressure, the application gauge shows how hard you are braking, and a low air warning you can see must come on before pressure falls below 60 psi. Antilock brakes (ABS) keep the wheels from locking, but they add no stopping power, and a yellow lamp warns you when the ABS is not working.
Key concepts
Governor cut-out and cut-in
The governor stops the compressor from pumping at the cut-out pressure (about 125 psi) and lets it pump again at the cut-in pressure (about 100 psi).
Memory tipOut at the top (125), in at the bottom (100).
Source: AAMVA-based state CDL manual, Section 5 · as of 2026
Safety relief valve
A valve in the first air tank that opens at about 150 psi to protect the system from too much pressure.
Memory tipIf it lets air out, the governor or system has a fault. Get it repaired.
Source: AAMVA-based state CDL manual, Section 5 · as of 2026
Air tank drains
Valves at the bottom of each tank that remove water and compressor oil. Manual drains are opened with a quarter-turn drain cock or a pull cable.
Memory tipManual drains: drain the tanks completely at the end of each day.
Alcohol evaporator
A device that puts alcohol into the air system to reduce the risk of ice in valves and other parts in cold weather.
Memory tipCheck and fill the alcohol container every day in cold weather.
Low air pressure warning
A required warning you can see (usually a red light, often with a buzzer, or a wig wag) that comes on before tank pressure falls below 60 psi. On older vehicles, it may come on at one-half of the governor cut-out pressure instead.
Memory tipWhen it comes on while driving, stop safely right away.
Source: AAMVA-based state CDL manual, Section 5 · as of 2026
Spring brakes
Brakes held off by air pressure and put on by strong springs when air is removed. On tractors and straight trucks they come fully on when pressure drops to about 20 to 45 psi.
Memory tipNever press the brake pedal while the spring brakes are on; the combined force can damage the brakes.
Source: AAMVA-based state CDL manual, Section 5 · as of 2026
Antilock brakes (ABS)
A system that keeps wheels from locking during hard braking. Required on air-brake truck tractors built on or after March 1, 1997, and on other air-brake vehicles such as trailers and converter dollies built on or after March 1, 1998.
Memory tipABS adds no stopping power. A yellow lamp means it is not working, but your normal brakes still work.
Source: 49 CFR 393.55 · as of 2026
Often tested
- Governor cut-out about 125 psi, cut-in about 100 psi; safety valve about 150 psi.
- Water and oil in the tanks can freeze and cause brake failure, so drain manual tanks every day.
- S-cam order: brake chamber, push rod, slack adjuster, camshaft, S-cam, shoes against the drum.
- Yellow diamond knob: pull out to set the parking (spring) brakes, push in to release.
- Low air warning before 60 psi; spring brakes come on at about 20 to 45 psi.
- ABS keeps the wheels from locking and adds no braking power. You may or may not stop faster with ABS.
Easy to confuse: Supply gauge vs. application gauge
| Item | Supply pressure gauge | Application pressure gauge |
|---|---|---|
| What it shows | Air pressure in the tanks | Air pressure you are applying to the brakes |
| On every air brake vehicle? | Yes (one per system, or two needles in dual systems) | Not on all vehicles |
| Warning sign | Falling pressure toward the low air warning | Needing more pressure to hold the same speed means the brakes are fading (or are out of adjustment, leaking, or have another problem) |
3 sample questions
Verified practice questions from this unit.
Question 1. Air brake systems are really three braking systems in one. What are they?
- ① Service brakes, engine brakes, and exhaust brakes
- ② Front brakes, rear brakes, and trailer brakes
- ③ Service brakes, parking brakes, and emergency brakes
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Answer: ③ Service brakes, parking brakes, and emergency brakes
Key point: Air brakes = service + parking + emergency
An air brake system combines three braking systems: the service brake (used in normal driving when you press the pedal), the parking brake (used when you park), and the emergency brake (uses parts of the service and parking brakes to stop the vehicle if the service brake system fails).
Wrong choices
- Service brakes, engine brakes, and exhaust brakes: Engine and exhaust brakes are speed retarders, not part of the three air brake systems.
- Front brakes, rear brakes, and trailer brakes: These are places where brakes are, not the three braking systems.
Question 2. What part controls when the air compressor pumps air into the air tanks?
- ① The safety relief valve
- ② The foot valve
- ③ The governor
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Answer: ③ The governor
Key point: Governor tells the compressor when to pump
The air compressor governor controls when the compressor pumps air into the tanks. When tank pressure is high enough, it stops the pumping; when pressure falls, it starts the pumping again.
Wrong choices
- The safety relief valve: The safety relief valve only opens to let air out if pressure gets too high.
- The foot valve: The foot valve (brake pedal) sends air to the brakes; it does not control the compressor.
Question 3. According to the CDL manual, at about what tank pressure does the governor stop the compressor from pumping air (cut-out)?
- ① About 100 psi
- ② About 125 psi
- ③ About 60 psi
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Answer: ② About 125 psi
Key point: Governor cut-out: about 125 psi
When air tank pressure rises to the cut-out level (around 125 psi, psi = pounds per square inch), the governor stops the compressor from pumping air.
Wrong choices
- About 100 psi: About 100 psi is the cut-in level, where pumping starts again.
- About 60 psi: 60 psi is where the low air pressure warning must come on, far below cut-out.