Air Brake System 2026: Parts, Diagram and Air Pressures

What are the parts of an air brake system?

Air compressor: driven by the engine, it squeezes air into a smaller space.
Governor: stops the compressor at cut-out pressure and starts it again at cut-in.
Air dryer: removes moisture before the air reaches the supply tank.
Supply (wet) tank: collects most of the moisture and oil.
One-way check valves: stop air flowing back out of the tanks.
Primary and secondary (dry) tanks: one for each circuit.
Safety valve: lets air out if the governor fails and pressure gets too high (normally at 150 psi).
Air pressure gauges and a low air pressure warning on the dash. Some vehicles have a wig wag, an arm that drops into the driver’s view when pressure is too low.
Brake pedal (foot valve), brake chambers, slack adjusters and the foundation brakes at the wheels.

Air pressures to know

Normal operating range: around 100 to 120 psi (pounds per square inch), or 690 to 828 kPa (kilopascals).
Cut-out is normally 20 to 25 psi above cut-in. Cut-out must never be above 145 psi, and cut-in never below 80 psi.
The low air warning must come on before either tank drops to 55 psi; most come on at 60 psi or higher.

These are the figures in Ontario’s Official Air Brake Handbook. Gauges may read in kPa or psi.

Draining tanks, air dryers and alcohol evaporators

Moisture and oil from the compressor settle in the tanks. They can make sludge that sticks valves, corrode parts, freeze in cold weather or fill the tank, and any of these can cause brake failure.
Drain the tanks regularly, supply tank first, and watch what comes out. Vehicles with an air dryer still need draining.
In cold climates an alcohol evaporator adds alcohol vapour to the air so moisture is less likely to freeze; use only products made for it. ICBC’s Driving Commercial Vehicles guide says the alcohol acts as an anti-freeze, lowering the freezing point of moisture in the system.

Spring brakes and caging bolts

Spring brakes are for parking and emergencies. A large coil spring applies them when air leaves the chamber, and air pressure holds them off. The spring brake control valve normally has a yellow, four-sided knob, and it pops out to apply the spring brakes when system pressure drops to about 60 psi. A technician can cage the spring with a caging bolt to move a vehicle in an emergency; a caged chamber won’t apply, so the vehicle needs repair right away. Never try to service a spring brake chamber yourself: the spring is under very high tension.

ICBC’s Driving Commercial Vehicles guide warns against compounding: making a heavy service brake application while the spring parking brakes are on adds the two forces together, which can damage slack adjusters, S-cams, brake chamber mounting bolts, shoes and drums. Release the spring brakes first.

Air brake system diagram

The official manuals have the diagrams. Ontario’s handbook ends with circuit diagrams of the air supply, a coach or bus, a towing vehicle and a trailer, and Nova Scotia’s Air Brake Manual opens with a full dual air brake system diagram, numbered to match its chapters.

Ontario handbook: Checklists, charts and circuit diagrams ↗Nova Scotia: Air Brake Manual (PDF) ↗Valves in the system →Trailer air brakes →

Sources: Ontario Official Air Brake Handbook · Nova Scotia Registry of Motor Vehicles · ICBC and 3 more. Checked October 2026.