3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response

Alberta Air Brake Q Endorsement

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1 Chapter 1: Alberta Q Endorsement Program and Air Brake Fundamentals

1.1 Alberta Q Endorsement Path, Course Requirements, and Knowledge Test Logistics 1.2 Licensing Scope, Farm Vehicle Exceptions, and the Technician Route 1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours

2 Chapter 2: The Supply Circuit: Compressor, Governor, Tanks, and Air Dryers

2.1 Air Compressors, Drive Systems, and Governor Operation 2.2 Supply and Service Tanks, Contamination, and Draining 2.3 Air Dryers, Air Brake Antifreeze, Safety Valves, and Check Valves

3 Chapter 3: Service Circuits, Control Valves, Gauges, and Low-Air Warnings

3.1 Dual Air Systems: Primary and Secondary Service Circuits 3.2 Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves 3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response

4 Chapter 4: The Park Circuit and Tractor/Trailer Circuits

4.1 Spring Parking Brakes, the Park Control Valve, Compounding, and Caging 4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands 4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers

5 Chapter 5: Air System Failures and Emergency Braking

5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits 5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks

6 Chapter 6: Foundation Brakes and Brake Adjustment

6.1 Foundation Brakes: S-Cam Drum, Disc, and Wedge Brakes 6.2 Brake Stroke, Adjustment Limits, and Stroke Indicators 6.3 Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters

7 Chapter 7: Braking Fundamentals, Driving Techniques, and Brake Management Systems

7.1 Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade 7.2 Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking 7.3 Antilock Brakes, Automatic Traction Control, and Stability Control

8 Chapter 8: Vehicle Inspections and the Air Brake Practical Test

8.1 Trip Inspection Law, Major and Minor Defects, and Roadside Enforcement 8.2 Inspection Set-Up and Recognizing Air Brake Defects 8.3 Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks 8.4 Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring

3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response

Key Takeaways Explain This Topic AI

The dash air pressure gauges show the pressure in the tractor's two service tanks (primary and secondary).

The low-air warning is set to come on if pressure drops below 60 PSI (414 kPa); manufacturers fit visual and audible alarms, and the sending units often trigger around 70 PSI.

On the trip inspection, the low-air warning should come on by 60 PSI as pressure drops and go out by 60 PSI during the build-up test.

CVSA does not put a vehicle out of service for its warning device if either the audible or the visual warning works; its minimum is 55 PSI or half the governor cut-out, whichever is less.

When the warning comes on while driving, make a steady brake application and stop safely; do not pump the brakes, because the spring brakes begin dragging around 55–60 PSI and are firmly applied by 45–50 PSI.

Last updated: September 2026

Air Pressure Gauges, Low-Air Warnings, and the Driver's Response

Quick Answer: The dash gauges show the pressure in the tractor's primary and secondary service tanks . Alberta's standard is that the low-air warning comes on by 60 PSI (414 kPa) : it activates if pressure drops below 60 PSI . Manufacturers fit both visual and audible alarms, and the sending units often trigger at about 70 PSI (483 kPa) . On the trip inspection, the warning should come on by 60 PSI as you reduce pressure and go out by 60 PSI as pressure builds. If it comes on while you are driving, make a steady brake application , stop in a safe place, and do not pump the brakes .

The air pressure gauges

Every air-brake vehicle has air pressure gauges on the dash. On a dual-system power unit, they measure the air in the two tractor service tanks : the primary and the secondary. The display may be two separate gauges, one gauge with two needles, or a digital display.

What normal looks like. With the engine running, both readings cycle between the governor's cut-in and cut-out pressures. Using the Alberta check values, that is roughly the low 100s up to 120–135 PSI . The two readings should behave alike.

What to watch for:

One gauge falling while the other holds points to a leak or failure in that circuit (see Chapter 5).

Both gauges falling and not recovering after dropping below cut-in points to a supply circuit problem. The brakes will work only on the air already stored.

Unusually low readings on both gauges after you charge a trailer can mean a trailer tank failure (Section 5.2).

Some vehicles also have an application gauge , which shows the pressure being delivered to the brakes during an application. A light stop uses little pressure. If you find you need noticeably more application pressure than usual to get the same slowing, suspect brake adjustment or fade and have it checked.

The low-air warning

Item Alberta course / Commercial Driver's Guide

When it must warn If pressure drops below 60 PSI (414 kPa) ; on the trip inspection it "should come on by 60 PSI"

What the manufacturer fits Both a visual and an audible alarm
Where the sending units usually trigger Around 70 PSI (483 kPa)
During the build-up test The warning light should go out by 60 PSI as pressure rises

The trip air brake inspection checks the warning twice. During the tractor protection check , with the trailer lines disconnected and pressure falling, the warning should come on by 60 PSI. It can also be checked as the park control valve is tested. During the supply circuit check , the warning should go out by 60 PSI as the compressor builds pressure. Sections 8.3 and 8.4 walk through both steps.

What enforcement looks for

The Commercial Vehicle Safety Alliance (CVSA) , the organization all enforcement agencies belong to, sets out-of-service standards for roadside inspections. The Alberta instructor material summarizes its low-air warning standard as follows:

The minimum operating point is 55 PSI or half of the governor cut-out pressure, whichever is less .

If either the audible or the visual warning is working, the vehicle is not placed out of service for the warning device.

For the driver's daily trip inspection, the National Safety Code schedule lists " low air warning system fails or system is activated " as a major defect . You may not drive the vehicle until it is repaired.

Why 60 PSI matters: what happens as pressure keeps falling

The warning gives you time before the park circuit starts to take over. The Alberta course gives the typical sequence as tank pressure falls:

Tank pressure What is happening
About 82–84 PSI (566–580 kPa) The park springs begin to expand

About 55–60 PSI (380–414 kPa) The springs have taken up the free play, and the linings start to drag on the drums

About 45–50 PSI (310–345 kPa) The park springs are applied firmly
Typically 20–45 PSI (138–310 kPa) The yellow park control valve pops out, which is its default "park" position

A spring brake application is only about half as strong as a full service application . Whether the wheels lock depends on the vehicle's weight, how sharply the brakes apply, and the available traction. ABS works only on the service brakes, so it will not help.

The correct response when the warning comes on

Believe it. Look at both gauges to see whether one circuit or both are dropping and how fast.

Apply the brakes steadily. The Alberta course is explicit: use a steady brake application to stop when a gauge is dropping and the low-air warning is on.

Do not pump the brakes. Each release throws away the air that was in the chambers. The Commercial Driver's Guide warns that pumping air brakes results in a loss of air pressure and, on a long downgrade, may leave too little air for the brakes to work properly.

Stop in a safe place , off the travelled lanes, before the spring brakes start to drag. Then secure the vehicle with the parking brakes.

Do not continue the trip. Find and repair the cause first. A low-air warning that is activated or has failed is a major defect.