Commercial Driver’s License (CDL) · CDL Manual Chapters
Air Brakes
On this page 6 sections
In 30 seconds
Air brakes use compressed air, not fluid, to push the shoes against the drums. A compressor makes the air, reservoirs store it, a foot valve meters it, and spring brakes hold the truck parked and clamp on by themselves if the air runs out. You must know the pressures where the warning sounds and the springs pop out, and you prove the system is tight with a timed leak test before every trip.
Why this matters
The Air Brakes knowledge test is 25 questions with 20 to pass, the seventh pre-trip step is the air-brake check you perform out loud, and failing the knowledge test or testing in a vehicle without full air brakes puts an L restriction on your license.
The college version
In normal terms
- Governor cuts in around 100 psi and cuts out around 125 psi, the low-air warning must come on before 60 psi, and the spring brakes pop out between 20 and 45 psi.
- Leak test, engine off and brakes released: lose no more than 2 psi per minute single vehicle, 3 combination; with the pedal held, 3 and 4.
- Pressure must build from 85 to 100 psi within 45 seconds, and you drain the tanks daily.
- In an emergency, stab brake: press fully, release when the wheels lock, press again when they roll.
Concepts in this outline
- Components: compressor, reservoirs, foot valve, spring brakes — Compressor pumps air, reservoirs store it, the foot valve meters it, and springs park the truck or stop it if air fails.
- Dual air systems — Two independent circuits with separate tanks so a leak in one still leaves working brakes on the other.
- Inspection/leak-down test procedures — Engine off, brakes released: lose no more than 2 psi per minute single, 3 combination; pedal held: 3 single, 4 combination.
- Low air pressure warning threshold — The light and buzzer must come on before pressure drops below 60 psi.
- Spring brake pop-out pressure — The parking brake knob pops out and the springs apply automatically, typically between 20 and 45 psi.
- Stab braking technique — Apply fully, release when the wheels lock, reapply as soon as they roll, and repeat until stopped.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Picture a bike pump, some air tanks, and a faucet. Air brakes use squeezed air, not liquid, to stop the truck. The compressor is the bike pump, run by the engine to fill the reservoirs. Those tanks hold air for several stops. The governor tells the pump when to work. It cuts in at about 100 psi and cuts out at about 125 psi. Psi measures how hard the air is pushing. The foot valve is the brake pedal, and it is the faucet. Press harder and more air flows.
Spring brakes are the safety net. Big springs in the rear brake chambers always try to squeeze the brakes shut. Air pressure is the only thing holding them open. Pull the yellow parking brake knob to let that air out, and the springs clamp on. If the air leaks away, the springs clamp on by themselves. That happens between 20 and 45 psi, on purpose. Before that, the low air pressure warning, a light and usually a buzzer, must come on. It must start before 60 psi, so you have time to pull over.
Trucks use a dual air system, two separate circuits with their own tanks, usually front and rear. A leak in one still leaves brakes on the other. Pressure should climb from 85 to 100 psi within about 45 seconds. Water collects in the tanks, so drain the reservoirs every day.
The leak-down test proves the system is tight. Chock the wheels, build full pressure, and shut off the engine. Release the parking brake and watch the gauge for a minute. A single vehicle may lose no more than 2 psi. A combination may lose 3. Then hold the pedal down and watch another minute. Now the limits are 3 psi single, 4 combination. Pump the pedal and confirm the warning comes on before 60 psi. Keep pumping until the knob pops out between 20 and 45 psi. Then rebuild pressure, roll at 5 miles per hour, and check the truck stops straight.
Three brakes share these parts. The service brake is your pedal. The parking brake is the yellow knob and its springs. The emergency brake is the springs when the air fails. Air takes about half a second to travel the lines. At 55 miles per hour that adds about 32 feet to a 290-foot stop. Controlled braking means squeezing firmly without locking the wheels. Stab braking means pressing fully, releasing when the wheels lock, and pressing again when they roll. Downhill, use a low gear so the engine slows you. Brake only in short, firm pushes, since hot brakes fade.
Worked example
Kwame reaches the seventh pre-trip step in a Class B dump truck, chocks the wheels, and builds pressure until the governor cuts out at 125 psi. Engine off, parking brake released, the gauge drops only 1 psi in a minute, under the 2-psi single-vehicle limit. Pedal held, it loses 2 psi the next minute, under the 3-psi limit. He pumps the pedal; at 65 psi the light and buzzer come on, before 60 as required, and at 35 psi the yellow knob pops out, inside the 20 to 45 range. He restarts and watches pressure climb from 85 to 100 psi in about 30 seconds, quicker than the 45-second limit. Rolling at 5 miles per hour, the truck stops straight.
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