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Spring brakes, parking brake controls and ABS

Air Brakes5 minManual section 5.1.14–5.1.16

Air can leak away, so parking and emergency brakes must be held on by springs. This lesson explains spring brakes and when they come on, the different parking brake controls, and antilock brakes (ABS): which vehicles need them, the warning lamps, and what ABS can and cannot do.

Spring brakes

All trucks, truck tractors and buses must have emergency brakes and parking brakes. These must be held on by mechanical force, since air can slowly leak out over time. Spring brakes are the usual way to do this.

While you drive, air pressure holds back powerful springs. Take the air away and the springs apply the brakes.

The air can be removed two ways: you use the parking brake control in the cab to let air out of the spring brakes, or a leak lets all the air out of the system. Either way, the springs apply the brakes.

On tractors and straight trucks, the spring brakes are fully on once air pressure falls into the 20 to 45 psi range (usually 20 to 30 psi).

Do not wait for the spring brakes to come on by themselves. As soon as the low air warning light and buzzer first come on, get the vehicle safely stopped right away, while the brakes are still under your control.

Spring brakes only stop well if the brakes are in adjustment. When adjustment is wrong, both the regular brakes and the emergency/parking brakes will perform poorly.

Parking brake controls

In newer air brake vehicles, the parking brake control is a yellow, diamond-shaped, push-pull knob. Pull it out to put the parking (spring) brakes on. Push it in to release them. On older vehicles, a lever may control the parking brakes. Use the parking brakes whenever you park.

Caution: do not press the brake pedal while the spring brakes are on. The springs and the air pressure pushing together could damage the brakes. Many systems are built to prevent this, but not all are, and the protection may not always work. Make it a habit.

Modulating control valve: in some vehicles, a handle on the dashboard applies the spring brakes gradually. It is spring-loaded, so you can feel the braking. The more you move the lever, the harder the spring brakes come on. This lets you control the spring brakes if the service brakes fail. To park, push the lever all the way and lock it there with the locking device.

Dual parking control valves: losing main air pressure makes the spring brakes come on. Some vehicles, buses for example, carry a separate air tank that can release the spring brakes so you can move the vehicle in an emergency.

  • One valve is a push-pull type, used to put on the spring brakes for parking.
  • The other valve is spring-loaded in the "out" position. Push it in and air from the separate tank releases the spring brakes so you can move. Let go and the spring brakes come on again.

The separate tank has only enough air to do this a few times. Plan carefully when you move, or you may be stopped in a dangerous place when the separate air supply runs out.

Antilock braking systems (ABS)

Who must have ABS: air-brake truck tractors built on or after March 1, 1997, and other air-brake vehicles (trucks, buses, trailers and converter dollies) made from March 1, 1998 on. Many older vehicles have ABS added voluntarily. Check the certification label for the date of manufacture.

ABS is a computer-controlled system that stops the wheels from locking up when you brake hard.

ABS vehicles have yellow malfunction lamps that tell you if something is not working:

  • Tractors, trucks and buses: yellow ABS lamp on the instrument panel.
  • Trailers: yellow ABS lamp on the left side, on the front or rear corner.
  • Dollies built on or after March 1, 1998: a lamp on the left side.

On newer vehicles, the lamp comes on at start-up for a bulb check, then goes out quickly. On older systems, it could stay on until you drive over 5 mph.

If the lamp stays on after the bulb check, or comes on while you are driving, you may have lost ABS control at one or more wheels.

For towed units built before ABS was required, it may be hard to tell. Check under the vehicle for an ECU (electronic control unit) and for wheel speed sensor wires running from the back of the brakes.

ABS is an add-on to your normal brakes. It neither adds to nor takes away from your normal braking ability. It works only when wheels are about to lock. ABS does not necessarily shorten your stopping distance, but it helps you keep control during hard braking.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 5.1.14–5.1.16

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

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Review questions with answers

  1. Why must parking and emergency brakes on trucks and buses be held on by mechanical force?
    1. Mechanical brakes are cheaper
    2. Air brakes cannot stop a heavy vehicle
    3. Air pressure can eventually leak away
    4. The law bans air for parking
    Show answer
    Air pressure can eventually leak away

    Air can leak away over time, so these brakes are held on mechanically, usually by spring brakes.

  2. What are spring brakes?
    1. Brakes put on by powerful springs when air pressure is removed
    2. Brakes that use springs to release the service brakes
    3. A type of hydraulic brake
    4. Brakes that only work in winter
    Show answer
    Brakes put on by powerful springs when air pressure is removed

    While driving, air holds the springs back. When air is removed, the springs apply the brakes.

  3. Tractor and straight truck spring brakes come fully on when air pressure drops to a range of:
    1. 60 to 80 psi
    2. 85 to 100 psi
    3. 100 to 125 psi
    4. 20 to 45 psi
    Show answer
    20 to 45 psi

    Spring brakes come fully on at 20 to 45 psi (typically 20 to 30 psi).

  4. The low air warning light and buzzer first come on while you are driving. What should you do?
    1. Keep driving until the spring brakes come on
    2. Get the vehicle safely stopped right away, while you still have control of the brakes
    3. Pump the brake pedal to build pressure
    4. Pull the parking brake knob immediately at highway speed
    Show answer
    Get the vehicle safely stopped right away, while you still have control of the brakes

    Do not wait for the spring brakes. Stop safely right away while you still control the brakes.

  5. If the brakes are out of adjustment, which brakes will not work right?
    1. Only the service brakes
    2. Only the parking brakes
    3. Both the regular brakes and the emergency/parking brakes
    4. Only the trailer brakes
    Show answer
    Both the regular brakes and the emergency/parking brakes

    Spring brake power depends on adjustment. Out-of-adjustment brakes affect both regular and emergency/parking braking.

  6. Besides using the parking brake control, what else will make spring brakes come on?
    1. A leak that causes all the air to be lost
    2. Driving over 55 mph
    3. Turning on the headlights
    4. Pressing the clutch
    Show answer
    A leak that causes all the air to be lost

    A leak that drains all the air will also let the springs apply the brakes.

  7. In newer air brake vehicles, the parking brake control is a:
    1. Round red knob you push in to apply
    2. Yellow, diamond-shaped knob you pull out to apply
    3. Blue lever you push down to apply
    4. Green button on the steering wheel
    Show answer
    Yellow, diamond-shaped knob you pull out to apply

    Newer vehicles use a yellow diamond-shaped push-pull knob: pull out to apply, push in to release.

  8. Why is it a bad idea to press the brake pedal while the spring brakes are applied?
    1. It drains the alcohol evaporator
    2. It turns off the ABS
    3. It releases the spring brakes
    4. The combined force of springs and air pressure could damage the brakes
    Show answer
    The combined force of springs and air pressure could damage the brakes

    Spring force plus air pressure together can damage the brakes. Not every system protects against this.

  9. A modulating control valve lets you:
    1. Drain the air tanks from the cab
    2. Change the governor cut-out pressure
    3. Apply the spring brakes gradually with a dash lever if the service brakes fail
    4. Turn off the low air warning
    Show answer
    Apply the spring brakes gradually with a dash lever if the service brakes fail

    The modulating valve is a spring-loaded handle that applies the spring brakes gradually; more lever movement means harder braking.

  10. When parking a vehicle with a modulating control valve, you should:
    1. Push the lever all the way and lock it with the locking device
    2. Leave the lever in the middle
    3. Push the brake pedal and hold it
    4. Release the lever fully
    Show answer
    Push the lever all the way and lock it with the locking device

    Move the lever all the way and lock it in place with the locking device.

  11. Some buses have dual parking control valves and a separate air tank. Why must you plan carefully when moving with this system after main air is lost?
    1. The separate tank refills too fast
    2. The separate tank has only enough air to release the spring brakes a few times
    3. It disables the steering
    4. It only works in reverse
    Show answer
    The separate tank has only enough air to release the spring brakes a few times

    There is only enough air in the separate tank for a few releases, so you could get stuck in a dangerous spot.

  12. With dual parking control valves, one valve is spring-loaded in the "out" position. When you push it in:
    1. The spring brakes come on
    2. The tanks drain
    3. The compressor stops
    4. Air from the separate tank releases the spring brakes so you can move
    Show answer
    Air from the separate tank releases the spring brakes so you can move

    Pushing it in uses separate-tank air to release the spring brakes; letting go applies them again.

  13. Truck tractors with air brakes must have ABS if they were built on or after:
    1. March 1, 1997
    2. March 1, 1998
    3. January 1, 1994
    4. January 1, 1975
    Show answer
    March 1, 1997

    Air-brake truck tractors built on or after March 1, 1997 must have ABS.

  14. Other air brake vehicles (trucks, buses, trailers, converter dollies) must have ABS if built on or after:
    1. March 1, 1997
    2. March 1, 1998
    3. March 1, 2000
    4. January 1, 1994
    Show answer
    March 1, 1998

    Trucks, buses, trailers and dollies with air brakes built on or after March 1, 1998 need ABS.

  15. How can you find out if your vehicle is equipped with antilock brakes?
    1. Pump the brakes and listen
    2. Look at the color of the parking knob
    3. Check the certification label for the date of manufacture, and look for the yellow ABS lamp
    4. Check the supply pressure gauge
    Show answer
    Check the certification label for the date of manufacture, and look for the yellow ABS lamp

    The certification label gives the build date; ABS vehicles also have yellow malfunction lamps.

  16. Where is the ABS malfunction lamp on a trailer?
    1. On the instrument panel of the tractor only
    2. On the right rear corner
    3. Inside the trailer near the doors
    4. On the left side, on the front or rear corner
    Show answer
    On the left side, on the front or rear corner

    Trailers have yellow ABS lamps on the left side, on the front or rear corner.

  17. What color are ABS malfunction lamps?
    1. Yellow
    2. Red
    3. Blue
    4. Green
    Show answer
    Yellow

    ABS malfunction lamps are yellow. The low-air warning light is usually red.

  18. On older ABS systems, the malfunction lamp may stay on after start-up until:
    1. The engine is warm
    2. Air pressure reaches 125 psi
    3. You are driving over 5 mph
    4. You apply the parking brake
    Show answer
    You are driving over 5 mph

    Newer lamps go out right after the bulb check; older ones may stay on until you drive over 5 mph.

  19. The yellow ABS lamp stays on after the bulb check. This means:
    1. ABS is working normally
    2. You may have lost ABS control at one or more wheels
    3. Your air pressure is below 55 psi
    4. The parking brake is on
    Show answer
    You may have lost ABS control at one or more wheels

    A lamp that stays on, or comes on while driving, may mean lost ABS control at one or more wheels.

  20. On an older trailer, how can you tell if it has ABS?
    1. It will have a blue parking knob
    2. It will have two air tanks
    3. It will have a wig wag
    4. Check underneath for an ECU and for wheel speed sensor wires running from the back of the brakes
    Show answer
    Check underneath for an ECU and for wheel speed sensor wires running from the back of the brakes

    Check under the trailer for an electronic control unit (ECU) and wheel speed sensor wiring.

  21. Which statement about ABS is true?
    1. It only works when wheels are about to lock up
    2. It doubles your normal braking power
    3. It always shortens stopping distance
    4. It replaces the service brakes
    Show answer
    It only works when wheels are about to lock up

    ABS is an addition to normal brakes; it does not increase or decrease braking power and acts only near lockup.

  22. True or False: ABS always shortens your stopping distance.
    1. True
    2. False
    Show answer
    False

    False. ABS does not necessarily shorten stopping distance, but it helps you keep control during hard braking.

  23. Converter dollies built on or after March 1, 1998 must have an ABS malfunction lamp located:
    1. On the left side
    2. On the right side
    3. Under the fifth wheel
    4. Inside the tractor cab only
    Show answer
    On the left side

    Dollies made from March 1, 1998 on must have the ABS lamp on the left side, like trailers.