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Gauges, low-air warning, valves and dual air systems

Air Brakes4 minManual section 5.1.9–5.1.13, 5.2

The gauges and warning devices tell you how much air you have. This lesson covers the supply and application gauges, the low air pressure warning (55 psi), the stop light switch, the front brake limiting valve, and how a dual air brake system splits the brakes into two halves.

Supply and application pressure gauges

Supply pressure gauge: every air brake vehicle has one, connected to the air tank. It tells you how much pressure is in the air tanks. A dual air system has a gauge for each half of the system, or one gauge with two needles.

Application pressure gauge: tells you how much air pressure is going to the brakes as you press the pedal. Not every vehicle has one.

If you need more and more application pressure to hold the same speed, the brakes are fading. Slow down and use a lower gear.

A need for more pressure can also mean out-of-adjustment brakes, air leaks, or mechanical trouble.

Low air pressure warning

A Low air pressure warning signal is required on air brake vehicles. A warning you can see must come on before tank pressure falls below 55 psi. On older vehicles, it comes on at one half of the governor cut-out pressure.

The warning is usually a red light, and there may also be a buzzer.

Another kind is the Wig wag: a mechanical arm that drops into your view when system pressure falls below 55 psi.

  • Automatic wig wag: rises out of view by itself when pressure goes back above 55 psi.
  • Manual reset wig wag: you must put it in the "out of view" position by hand, and it will not stay there until pressure is above 55 psi.

On large buses, it is common for low-pressure warnings to signal at 80–85 psi.

Stop light switch and front brake limiting valve

Drivers behind you must be warned when you brake. The air brake system uses a Stop light switch: an electric switch worked by air pressure that lights the brake lights whenever you apply the air brakes.

Some older vehicles (built before 1975) have a Front brake limiting valve with a cab control marked "normal" and "slippery." In the "slippery" position, the valve cuts normal air pressure to the front brakes by half.

These valves were meant to reduce front-wheel skids on slippery roads, but they actually reduce the vehicle's stopping power. Front wheel braking is good under all conditions. Tests show front-wheel skids from braking are not likely even on ice. Keep the control in "normal" for normal stopping power.

Many vehicles have automatic front wheel limiting valves. They reduce air to the front brakes except when you brake very hard (55 psi or more application pressure). The driver cannot control these valves.

Dual air brake systems

Most heavy-duty vehicles use a Dual air brake system for safety. It is two separate air brake systems that share one set of brake controls. Each half has its own tanks, hoses and lines.

  • The Primary system usually works the regular brakes on the rear axle or axles.
  • The Secondary system usually works the regular brakes on the front axle (and possibly one rear axle).

Both systems supply air to the trailer, if there is one.

Before driving, let the compressor build at least 100 psi in the primary system and in the secondary system. Keep an eye on both gauges (or both needles on one gauge), and on the low-air warning light and buzzer.

The warning light and buzzer should turn off once both systems climb to a pressure the manufacturer sets. That value must be greater than 55 psi.

The warning should come on before either system falls below 55 psi. If it comes on while you drive, stop right away and park safely.

Why: if one system is very low, either the front or the rear brakes are not working fully, so your stopping distance gets longer. Get the vehicle safely stopped and have the air brakes repaired.

Check your state

This course teaches the national rule: the low-air warning must come on before pressure falls below 55 psi (the Utah, Texas and Kansas manuals all say 55). About a dozen state manuals say 60 psi instead (for example New York, North Carolina, Michigan, Georgia, Washington and Wisconsin). Your state's page on 7CDL shows which number its manual uses. On the test, choose 55 psi unless your own state manual says otherwise. Check your state manual.

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: 5.1.9–5.1.13, 5.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

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

Preparing your questions…

Review questions with answers

  1. What does the supply pressure gauge tell you?
    1. How much pressure you are applying to the brakes
    2. The engine oil pressure
    3. How much pressure is in the air tanks
    4. The tire pressure
    Show answer
    How much pressure is in the air tanks

    The supply pressure gauge, connected to the air tank, shows the pressure in the tanks.

  2. A vehicle with a dual air brake system will have:
    1. A supply gauge for each half of the system, or one gauge with two needles
    2. No supply pressure gauge
    3. Only an application pressure gauge
    4. A gauge on the trailer only
    Show answer
    A supply gauge for each half of the system, or one gauge with two needles

    Dual systems have a gauge for each half, or a single gauge with two needles.

  3. The application pressure gauge shows:
    1. The air pressure in the tanks
    2. How much air pressure you are applying to the brakes
    3. The governor cut-out setting
    4. The air pressure in the tires
    Show answer
    How much air pressure you are applying to the brakes

    The application gauge shows the pressure you are applying to the brakes. Not all vehicles have one.

  4. You need more and more application pressure to hold the same speed on a downgrade. This means:
    1. The compressor is pumping too much
    2. The tanks are overfilled
    3. The ABS is working
    4. The brakes are fading; slow down and use a lower gear
    Show answer
    The brakes are fading; slow down and use a lower gear

    Rising application pressure for the same speed means brake fade. Slow down and use a lower gear.

  5. Besides brake fade, needing more application pressure can be caused by:
    1. Brakes out of adjustment, air leaks or mechanical problems
    2. A full fuel tank
    3. Cold tires
    4. Using a high gear
    Show answer
    Brakes out of adjustment, air leaks or mechanical problems

    Brakes out of adjustment, air leaks or mechanical problems can also require more pressure.

  6. A low air pressure warning signal must come on before tank pressure falls below:
    1. 20 psi
    2. 100 psi
    3. 55 psi
    4. 85 psi
    Show answer
    55 psi

    The visible warning must come on before pressure falls below 55 psi (half of governor cut-out on older vehicles).

  7. True or False: Every vehicle with air brakes is required to have a low air pressure warning signal.
    1. True
    2. False
    Show answer
    True

    True. A low air pressure warning is required on air brake vehicles.

  8. On older vehicles, the low air warning may come on at:
    1. Twice the cut-in pressure
    2. One half of the governor cut-out pressure
    3. The safety valve setting
    4. 20 psi
    Show answer
    One half of the governor cut-out pressure

    On older vehicles the warning comes on at one half of the compressor governor cut-out pressure.

  9. What is a "wig wag"?
    1. A light that flashes when you brake
    2. A valve that drains the tanks
    3. A gauge with two needles
    4. A mechanical arm that drops into your view when pressure falls below 55 psi
    Show answer
    A mechanical arm that drops into your view when pressure falls below 55 psi

    A wig wag is a low-air warning arm that drops into view below 55 psi.

  10. A manual reset wig wag:
    1. Rises out of view by itself above 55 psi
    2. Has no connection to air pressure
    3. Must be put out of view by hand and will not stay there until pressure is above 55 psi
    4. Only works on trailers
    Show answer
    Must be put out of view by hand and will not stay there until pressure is above 55 psi

    The manual type must be reset by hand and will not stay up until pressure is over 55 psi.

  11. On large buses, low pressure warning devices commonly signal at:
    1. 55–60 psi
    2. 80–85 psi
    3. 100–110 psi
    4. 20–45 psi
    Show answer
    80–85 psi

    It is common for large buses to have low-pressure warnings at 80–85 psi.

  12. How does an air brake system turn on the brake lights?
    1. With an electric switch worked by air pressure
    2. With a cable from the brake pedal
    3. With the ABS computer only
    4. The driver turns them on with a dash switch
    Show answer
    With an electric switch worked by air pressure

    The stop light switch is an electric switch worked by air pressure.

  13. On an older vehicle with a front brake limiting valve, the "slippery" position:
    1. Doubles front brake pressure
    2. Turns on the ABS
    3. Locks the rear wheels
    4. Cuts normal air pressure to the front brakes by half
    Show answer
    Cuts normal air pressure to the front brakes by half

    In "slippery" the limiting valve cuts front brake air pressure by half.

  14. True or False: Front wheel braking is good under all conditions.
    1. True
    2. False
    Show answer
    True

    True. Tests show front-wheel skids from braking are unlikely even on ice. Keep the limiting valve on "normal."

  15. Automatic front wheel limiting valves reduce air to the front brakes except when:
    1. The road is wet
    2. The brakes are applied very hard, at 55 psi or more application pressure
    3. The vehicle is empty
    4. The driver selects "slippery"
    Show answer
    The brakes are applied very hard, at 55 psi or more application pressure

    Automatic valves reduce front air except during very hard braking (55 psi or more). The driver cannot control them.

  16. Which vehicles may have a front brake limiting valve with a cab control?
    1. All vehicles built after 2000
    2. Only school buses
    3. Some older vehicles made before 1975
    4. Only trailers
    Show answer
    Some older vehicles made before 1975

    Some vehicles made before 1975 have a front brake limiting valve with a "normal/slippery" control.

  17. What is a dual air brake system?
    1. An air system plus a hydraulic system
    2. Two separate air brake systems that use a single set of brake controls
    3. Two compressors feeding one tank
    4. A system with brakes on both tractor and trailer
    Show answer
    Two separate air brake systems that use a single set of brake controls

    A dual system is two separate air systems, each with its own tanks, hoses and lines, sharing one set of controls.

  18. In a typical dual air brake system, the primary system operates the brakes on:
    1. The rear axle or axles
    2. The front axle only
    3. The trailer only
    4. The parking brakes only
    Show answer
    The rear axle or axles

    The primary system usually works the rear axle brakes; the secondary works the front axle (and possibly one rear axle).

  19. Before driving a vehicle with a dual air system, let the compressor build up at least:
    1. 55 psi in the primary system
    2. 125 psi in one system
    3. 85 psi in either system
    4. 100 psi in both the primary and secondary systems
    Show answer
    100 psi in both the primary and secondary systems

    Wait for at least 100 psi in both systems before driving.

  20. In a dual air system, the low air warning light and buzzer should shut off when pressure in both systems rises above a value set by the maker. That value must be:
    1. Less than 20 psi
    2. Exactly 100 psi
    3. Greater than 55 psi
    4. Greater than 150 psi
    Show answer
    Greater than 55 psi

    The shut-off value is set by the manufacturer and must be greater than 55 psi.

  21. While driving a vehicle with dual air brakes, the low air warning comes on. One system is very low. What will happen and what should you do?
    1. Nothing changes; keep driving to your stop
    2. Front or rear brakes will not work fully, so stopping takes longer; stop safely right away
    3. The ABS will take over; slow down a little
    4. Both systems will refill; pump the brakes
    Show answer
    Front or rear brakes will not work fully, so stopping takes longer; stop safely right away

    If one half is low, either front or rear brakes are not fully working and stopping distance grows. Stop safely and get it fixed.

  22. In a dual air brake system with a trailer, which system supplies air to the trailer?
    1. Both the primary and secondary systems
    2. Only the primary system
    3. Only the secondary system
    4. Neither; the trailer has its own compressor
    Show answer
    Both the primary and secondary systems

    Both systems supply air to the trailer, if there is one.