Study guide
1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours
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
1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours
Key Takeaways Explain This Topic AI
An engine-driven compressor builds air pressure that travels through hoses and valves to apply the brakes; all air-brake vehicles have air pressure gauges on the dash.
1 PSI equals 6.895 kPa, 1 kPa equals 0.145 PSI, and 1 bar equals about 14.5 PSI or 100 kPa, so 60 PSI is about 414 kPa.
Stand clear when draining tanks and when air dryers purge, and when disconnecting a charged air line turn your head away and cover the coupler with your free hand.
In the Alberta course schematics, the supply circuit is blue, the primary service circuit green, the secondary service circuit red, and the park circuit orange.
Solid colours on the schematic are sections filled from the compressor that stay filled; dashed colours are sections the driver controls.
Last updated: September 2026
Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours
Quick Answer: An air brake system uses an engine-driven compressor to build air pressure, which is stored in tanks and routed through hoses and valves to brake chambers that apply the brakes. Pressure is measured in pounds per square inch (PSI) , kilopascals (kPa) , or bar : 1 PSI = 6.895 kPa , and 1 bar = about 14.5 PSI = 100 kPa . Compressed air and compressed brake springs can injure you, so stand clear of draining tanks and purging dryers , and turn your head away and cover the coupler when you disconnect a charged line. The Alberta course schematics use colour codes: blue = supply , green = primary service , red = secondary service , and orange = park .
How compressed air stops a vehicle
Air brakes use compressed air to slow or stop a vehicle. The compressor draws in atmospheric air, squeezes it into a much smaller volume, and pushes it into storage tanks (reservoirs). When the driver applies the brakes, valves release some of that stored air into brake chambers at the wheels. There it pushes on a diaphragm and turns air pressure into mechanical force at the foundation brakes.
Why do heavy vehicles use air instead of hydraulic fluid alone?
The supply is unlimited. The system makes its own working fluid from the atmosphere, so a small leak does not use up a finite reservoir the way a hydraulic leak does.
It extends to trailers. Air can be sent through flexible lines and couplers to one or more trailers, and those lines can be connected and disconnected in the field.
It fails safe for parking and emergencies. Spring parking brakes are held off by air. If the air is lost, the springs apply the brakes.
It multiplies force. Brake chambers turn modest air pressure into large pushrod forces, as Chapter 3 shows.
On most trucks, the air tanks and brake chambers are easy to see. On vehicles such as fire trucks and motorhomes, the body can hide them, and you may need to get very low to the ground to inspect them. Every air-brake vehicle has air pressure gauges on the dash .
Pressure units and conversions
Vehicle gauges may use PSI, kPa or bar. Read the units actually marked on the gauge. One bar is 100 kPa; a standard atmosphere is a different unit, equal to 101.325 kPa. The Alberta materials give most numbers in both PSI and kPa. For example, the low-air warning comes on by 60 PSI (414 kPa) , and the governor cuts out at 120–135 PSI (828–931 kPa) .
- Conversion Value
- 1 PSI 6.895 kPa, or 0.0689 bar
- 1 kPa 0.145 PSI, or 0.01 bar
- 1 bar about 14.5 PSI, or exactly 100 kPa
- 1 standard atmosphere about 14.7 PSI, or exactly 101.325 kPa
- Worked examples
60 PSI × 6.895 = 413.7 kPa , rounded to 414 kPa (the low-air warning point).
120 PSI × 6.895 = 827.4 kPa , or 827 kPa to the nearest whole kPa . The Alberta course separately uses the approximate paired value 120 PSI / 828 kPa for the bottom of the cut-out range; 828 is not the nearest-whole result of this multiplication.
50 PSI × 6.895 = 344.8 kPa , rounded to 345 kPa (the start of the build-up test).
700 kPa × 0.145 = 101.5 PSI .
If a question gives only kPa, convert it before comparing it with a PSI rule you have memorized.
The dangers of compressed air and compressed springs
The course repeats safety warnings because stored energy in an air brake system can cause serious injury:
Valve exhausts. Control valves and quick release valves exhaust forcefully when the service brakes are released or the park brakes are applied. The driver is usually in the cab when this happens, but anyone working near those valves must expect the blast.
Draining tanks. Stand clear of the discharge when you open a drain valve or pull a drain lanyard, because water, oil, and debris come out under pressure.
Air dryer purges. Air dryers purge (back-flush) powerfully at governor cut-out and can throw contaminants.
Disconnecting charged lines. When you uncouple a charged glad hand, turn your head away and cover the coupler with your free hand . Keep your hand between the coupler and your face so air and debris cannot blow into your eyes.
Maintenance work. Drain the tanks before any air system work, and wear proper personal protective equipment (PPE).
Park brake springs. The spring inside a spring brake chamber is compressed with great force. A corroded or damaged chamber can release it violently, so never try to take a spring chamber apart.
The course also stresses that drivers are always responsible for the performance of their brake systems . The course teaches how the system works, how to inspect it, and how to handle failures. It is not a repair course, and repairs need proper mechanical expertise.
Reading the course schematics
The Alberta course builds everything on colour-coded schematics, and the foldout in the student manual uses the same scheme:
- Colour Circuit What it feeds
- Blue Supply circuit Compressor, governor, air dryer, and the supply (wet) tank
- Green Primary service circuit On a truck or tractor, normally the drive axle(s)
- Red Secondary service circuit Normally the steering axle
- Orange Park circuit Spring parking brakes
Two drawing conventions matter:
Solid colours are sections filled from the compressor that stay filled , such as tanks and supply lines.
Dashed colours are sections the driver controls , such as service application air that is present only while the brake is applied, or the park circuit as it is released.
Trailer circuits take the colour of the tractor system that supplies them. The two air lines to a trailer are drawn red (supply/emergency) and blue (service) to match the glad hand lines on real trucks. Chapter 4 explains those lines. A separate colour scheme recommended by the Technology and Maintenance Council of the American Trucking Associations is also in use. Some manufacturers follow it and others use the red/green/orange system, so never identify a line on a real truck by colour alone. Trace it.