Air brakes

By the end of this topic you can

• You can name the main parts of an air brake system and say what each does.
• You can explain the difference between the service, parking and emergency brakes.
• You can describe what happens as air pressure drops and how spring brakes respond.
• You can carry out the in-cab air brake checks in your state manual.
• You can brake safely on long downgrades without overheating the brakes.

What you need to know and understand

• An air brake system is really three systems: service brakes for normal stops, parking brakes, and emergency brakes that use parts of the other two.
• The compressor fills the air tanks, and the governor tells it when to start and stop pumping. Many state manuals give typical figures of about 100 psi to cut in and about 125 psi to cut out.
• Spring brakes are held off by air pressure. If pressure falls too low they come on automatically; state manuals commonly give a range of about 20 to 45 psi.
• A low air pressure warning must come on before pressure falls to an unsafe level; the exact figure to learn is in your state CDL manual.
• Air brakes have brake lag: a short delay while air moves through the lines, so stopping distance is longer than you expect.
• Water and oil collect in the air tanks and must be drained, either by the driver or by automatic drain valves.

Inside the system

• Air compressor and governor: build and control the pressure.
• Air tanks with drain valves: store the air and let you remove moisture and oil.
• Brake pedal (foot valve): the harder you press, the more air goes to the brakes.
• Brake chambers, slack adjusters and S-cams or disc parts: turn air pressure into braking force at each wheel.
• Gauges and warning devices: supply pressure gauge, application pressure gauge and low-pressure warning.
• Spring brakes and the parking brake control, usually a yellow diamond-shaped knob.

The air brake check, step by step

Every state manual has its own version of this check, often around seven steps, and examiners expect it on the skills test. A typical sequence is:

1. Chock the wheels, start the engine and watch pressure build into the normal range within the time your manual states.
2. Note when the governor cuts out and, after pumping some air down, when it cuts back in.
3. With the engine off and brakes released, check the air leakage rate against your manual's limit.
4. Apply the foot brake firmly and check the leakage rate again.
5. Fan the pedal to lower the pressure and confirm the low air warning comes on at the right point.
6. Keep fanning and confirm the parking brake knob pops out as the spring brakes apply.
7. Test the parking brake and then the service brake by trying to move gently against them.

Braking on the road

Plan stops early because of brake lag. On a long downgrade, choose a low gear before you start down and let the engine do most of the work. When you do brake, many manuals teach snub braking: brake firmly to drop a few mph below your safe speed, release, and repeat, rather than riding the brakes all the way down, which overheats them and causes brake fade.

If the low air warning comes on while driving, stop safely as soon as you can while you still have control of the brakes. Do not keep driving and hope it builds up again.

Common mistakes

• Pumping air brakes like old car brakes. Fanning the pedal wastes air pressure.
• Riding the brakes on a downgrade. Pick a low gear first and use controlled snub braking.
• Skipping tank draining on vehicles with manual drains, letting water freeze in the lines in winter.
• Ignoring a low air warning. It means stop safely now, not later.
• Memorizing psi numbers from another state's manual. Learn the figures in your own state CDL manual.

Test yourself: Air brakes

Answer each question, then read the explanation. Questions come in a random order each time.

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