Commercial vehicles differ from regular passenger vehicles in that most are equipped with air brakes.

Air brakes work by charging a supply reservoir with compressed air. The air travels from the supply reservoir through both the primary and secondary service reservoirs. A "one-way check valve" at the entrance of each of the two service reservoirs stops the compressed air from going back into the supply reservoir. This means that once the air is "charged" enough air will stay in the tanks to stop the vehicle if the supply system fails.

If a commercial vehicle has three axles, the air from the primary service reservoir is used to apply the brakes on two of those axles. The air in the secondary tank is used for brakes on the third and remaining axle.

In two axle units "primary air" is used for the rear axle brakes and "secondary" for the front.

As the driver applies the brakes, compressed air goes into the brake chamber.

The pressure of the air on the chamber diaphragms causes the push rods to move outwards. This, in turn, moves the slack adjusters.

As the slack adjusters move, they rotate the brake cams, forcing the brake shoes against the drum to cause friction and stop the vehicle.

As the driver releases the brake, the air in the brake chamber escapes, releasing the brakes.

Pre-Trip Air Brake Inspections

If a commercial vehicle is regularly maintained, it will be safer to drive and more economical to operate. The driver is the key link in the maintenance chain of an air brake unit.

It is in the driver's best interest to be alert and know about how the air brake system works. Driver's should report and repair any air brake problem quickly.

Before taking off on a trip, drivers should inspect their air brake system to ensure that:

• The compressor is operating efficiently, and the belts are in good condition and have been adjusted.
• The lines, fittings, hoses and trailer couplers are in good condition.
• The visible part of the brake drums don't have any cracks.
• The brake linings are in good condition.

The Alberta Office of Traffic Safety, Alberta Transportation
Main Floor, Twin Atria, 4999 - 98 Avenue
Edmonton, Alberta T6B 2X3
Phone: (780) 422-8839
Fax: (780) 422-3682
E-mail: debbie.lamoureux@gov.ab.ca
Outside Edmonton call 310-0000 and ask for 422-8839.