1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours

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

North American trucks traditionally show pressure in PSI. Some manufacturers use kPa, and European vehicles use bar (one bar is roughly one atmosphere). 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).

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, rounded to 828 kPa (the bottom of the cut-out range).
• 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:

1. 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.
2. 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.
3. Air dryer purges. Air dryers purge (back-flush) powerfully at governor cut-out and can throw contaminants.
4. 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.
5. Maintenance work. Drain the tanks before any air system work, and wear proper personal protective equipment (PPE).
6. 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:

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.