46 topics · Updated 2026-09-17
Service and park brakes operate the same on trailers as on power units, and relays and control lines also work the same. Air pressure is provided by the tow vehicle, so the trailer has no supply circuit.
Glad hand couplers connect the air supply and brake signal lines to the trailer. They must be fully inspected before connecting; soft or degraded seals leak, and damaged couplers may not press together. Drivers should carry spare seals; the thick side goes into the coupler.
The tractor's protection system has two components: the trailer air supply valve (red button) controlling the supply line, and the tractor protection valve which uses supply line pressure to open the service line. Enforcement officers check both during CVSA Level 1 inspections; failure means out-of-service.
Pulling the yellow park control valve activates all spring park brakes. In two-button systems, the red trailer supply valve pops out when yellow is pulled. Drivers often hold the red button in while pulling yellow to keep trailer brakes released and prevent linings freezing to drums.
Service couplers are marked 'Service' or 'S' and traditionally connect to the passenger side. Hand valves send control air through the trailer service (blue) line to the trailer relays. Hand valves apply only the trailer service brakes and are not to be used for parking.
Older air pressure park brakes supply service tank air to the service chambers, creating a very powerful emergency brake twice as strong as spring park brakes, but with increased lockup risk. The problem is they do not stay on—as air drains, brakes release. Trailers must be chocked when uncoupled.
Service and park brakes, relays, and control lines operate the same on trailers as on power units. Air pressure is provided by the tow vehicle, so the trailer has no supply circuit of its own.
The tractor's protection system has two components: the trailer supply valve (red button) controlling the supply line, and the tractor protection valve using supply line pressure to open the service line. Enforcement checks both parts.
In two-button systems the red trailer supply valve automatically pops out when the yellow park control valve is pulled. Drivers often hold the red button in while pulling yellow to keep trailer brakes released in cold weather to prevent freezing to drums.
Applying the foot brake activates both tractor and trailer service brakes together. The hand valve applies only the trailer service brakes.
Air pressure park brakes on older trailers act as a powerful emergency brake (twice as strong as spring park brakes) but do not stay on—they release as air drains out. This requires special operating procedures.
Trailer air valves are expensive and must be kept clean. Dirt from bobtails migrates into trailer valves. A quick discharge of air pressure before connecting helps blow out contamination. Leave couplers open, pull the hand valve, hold the red dash valve in, then release both and connect.
All supply couplers have the word 'Emergency' or a large 'E' on them and typically couple to the left (driver) side by convention. The air pressure supplying the trailer tanks comes from the higher of the two tractor service tanks.
Prior to 1976, trailers used air park brakes, still found on some converter dollies and heavy hauling equipment. As of January 1, 1976, Section 121 of the Canadian Motor Vehicle Safety Standards (CMVSS) requires spring park brakes on trailers.
Service couplers say Service or have a large S and traditionally connect to the passenger side. The hand valve sends control air through the trailer service (blue) line to the trailer relays and applies only the trailer service brakes.
Prior to 1976, trailers used air park brakes; this system remains on many converter dollies and specialized heavy-haul equipment. As of January 1, 1976, CMVSS Section 121 requires spring park brakes on trailers.
The Commercial Vehicle Safety Regulation of the Traffic Safety Act describes requirements for commercial vehicles. Sections 9 through 16 outline vehicle inspection requirements. Section 10 covers the vehicle trip inspection valid for 24 hours, and Section 12 provides requirements for completing trip inspection reports. Beyond regulatory minimums, industry best practice is to inspect vehicles before, during and after each trip, as each inspection has a different purpose.
Minor defects must be reported before the next required inspection, and the vehicle may continue to be driven. Major defects require the vehicle to be placed out-of-service immediately, even in the middle of a driver's shift. If any system failures affect braking ability, the vehicle must be placed out-of-service until proper repairs are made.
There is no mandatory order for a vehicle trip inspection, but every driver must incorporate the air brake pre-trip into their normal inspection process. The recommended practical sequence starts with air tanks full, cycles pressure down with the tractor protection system check, back up with the air supply circuit check, then tests for leaks with the system full again.
The tractor protection system is checked in two parts: the trailer air supply valve (red button) with the service line open during a service brake application, and the park control valve (yellow button - not part of the tractor protection system but inspected together for efficiency). If the trailer supply valve trips immediately after air lines separate, return to the cab and firmly apply the foot valve to confirm the tractor protection valve closed the trailer service line. When disconnecting a live air line, place your hand between the glad hand couplers and your face to prevent debris being blown into the face, and wear proper PPE.
Broken push rods prevent both service and park applications from delivering brake power; they are not always obvious and must be looked at directly. Clevis pin failures occur when cotter pins shear and the clevis pin works out; brakes fail when the clevis fully separates. Separated brake chambers deliver no application at the foundation brake. Loose brake chambers reduce force through chamber deflection - inspect mounting fasteners and brackets. A corroded park spring chamber is extremely dangerous because the compressed park spring can escape with tremendous force; any corrosion or damage in chambers or clamp rings must be repaired immediately by certified heavy equipment technicians.
Brake lining failure reduces friction surface area and brake force. Extremely worn linings can cause a 'cammed over' condition where the lining becomes so thin that rollers pass the largest radius of the s-cam lobes, making brakes ineffective. CVSA standards require at least 1/4 inch (7 mm) lining thickness. Cracked drums result from repeated overheating and cooling; cracks start on the friction surface and work outward, then centrifugal force widens them until the drum separates - separated pieces become dangerous projectiles while the vehicle is moving.
In-service brake checks examine brake systems monitored while the vehicle is in motion. The en-route inspection is ideally done every few hours to confirm the vehicle can remain in service. End-of-shift inspections are important for the success of the following shift. Brake check pullouts have white information signs (all white signs are requirements) advising what to inspect and information about the grade ahead.
The low air pressure warning light is checked by pushing in the yellow park control valve and reducing air pressure until the yellow button pops out. Sometimes the yellow park control valve pops out below 20 PSI (138 kPa), even in new vehicles. Ensure the park control valve is popped out to its default position before the tanks are emptied.
The air supply circuit inspection includes the air compressor, governor, and low air pressure warning device. The system leaks procedure checks the entire air brake system for pressure leaks in one operation; if leaks are found, they must be isolated by subsystem. The only air line not filled during the leak procedure is the control line from the hand valve to the double check valve (on vehicles equipped with hand valves).
Oil saturation is caused by leaking wheel seals. Oil leaking while moving streaks in radius lines around the inner sidewall of the inner tire; oil leaking while parked drains down the inner sidewall and puddles on the ground. Slack adjuster failures can occur when a retaining snap ring separates from the s-cam shaft end and the slack adjuster works off the splines - brakes apply normally until the moment it separates, after which there is no foundation brake application.
Chaffed air lines result from lines rubbing on nearby parts; chaffing causes no performance loss until the hose ruptures, causing circuit failure. Hose bulges (from internal failure or chaffing) will rupture and cause circuit failure. Kinked plastic air lines are permanently damaged and must be replaced; a kinked rubber line can suffer internal damage leading to bulging. Fittings from different systems are not compatible and must never be mixed - use the correct fitting system for the hose. Blocking off air lines to a failed component reduces braking capacity and is never acceptable; the vehicle must be placed out-of-service.
A loose air tank does not cause a failure until it separates from the circuit it supplies, which then causes an immediate circuit failure. During inspection, push on each end of the air tanks to confirm they are securely mounted.
To test the supply circuit, start the engine and run at a fast idle around 1200 RPM. Perform the compressor build-up test; pressure must build from 50 to 90 PSI (345 to 621 kPa) within 3 minutes. Build air pressure to maximum to confirm the governor cut-out is at 120-135 PSI (828-931 kPa). Pump service brakes to reduce air pressure until the governor cuts in; confirm the cut-in is 20-25 PSI (138-172 kPa) less than the cut-out pressure.
The Notice of Air Brake Program Completion is issued after successful completion of the classroom session, practical training session, and practical test. It authorizes Registry Agents to provide the knowledge test. The original is presented to a Registry Agent with government issued photo ID. Tampered forms will not be accepted. It has an audit control number monitored by Alberta Transportation.
All student records must be retained for a minimum of two years. The completed Air Brake Student Record form(s), Air Brake Practical Test form(s), and the training school/agency copy of the Notice of Air Brake Program Completion form must be filed together in the student file. Documents must be available for inspection by Alberta Transportation.
Coupling requires chocking the trailer, checking the fifth wheel jaws are open, inspecting components, backing to match heights, connecting air, and verifying a solid hook-up.
Uncoupling requires parking in a straight line, setting brakes, chocking wheels, lowering landing gear, disconnecting lines, unlocking the fifth wheel, and slowly pulling clear.
Pintle hitch attachments have specific coupling and uncoupling steps involving safety cables, pins, and the pintle hook locking mechanism.
A controlled crossing has a flag person, stop sign, crossing gate, or an electric or mechanical signalling device. All vehicles are required to stop at controlled railway crossings if signalled to do so.
Some vehicles are required by law to stop at all uncontrolled railway crossings: school buses; vehicles carrying explosives as cargo or part of cargo; and vehicles designated for carrying flammable liquids or gas, whether loaded or empty. Obey any police officer or identified railway flag person, and be aware municipalities may have additional bylaws.
Coupling involves chocking the wheel, checking fifth wheel jaws are unlocked, inspecting apron/king pin/collar, backing in line and stopping before contact, matching heights, connecting air lines (if no spring brakes), setting trailer brakes, backing to connect, and performing a firm tug test.
After coupling, visually check that the fifth wheel jaws are properly locked around the trailer king pin, raise the landing gear, hook up the electrical line, and remove wheel chocks. Some older trailers may not have spring brakes.
Uncoupling involves parking straight and setting park brakes, chocking wheels, lowering landing gear, disconnecting electrical/air lines, unlocking the fifth wheel, securing lines to the tractor, moving forward slowly to clear the fifth wheel, verifying ground/landing gear support, and moving forward to fully clear.
The most important task in the coupling procedure is to physically and visually check all connections. Failure to do so may cause a serious incident.
Pintle hitch procedures require setting parking brakes, chocking wheels, disconnecting/connecting air lines, electrical and hoses, handling safety cables/chains and safety pin, and operating the pintle hook safety latch. Ensure the pintle hook and eye have no cracks or excessive wear.
If carrying passengers on a bus and you discover a fire or danger of fire, stop immediately in a safe location, get passengers off the bus, and move them to a safe spot at least 35 metres (115 feet) from the vehicle. Portable fire extinguishers are carried in the driver's compartment of most commercial vehicles.
Do not try to fight a fire beyond your capability or the extinguisher's. Depending on type and size, you have only eight to ten seconds of chemical discharge. Basic use: remove from bracket, pull the safety pin (breaking the seal), approach from upwind if possible, and hold upright.
Remember PASS: PULL the pin and point the nozzle away from you; AIM low and direct at the base of the fire; SQUEEZE the handle slowly and evenly until the fire is out or the extinguisher is empty; SWEEP side-to-side, starting at one side and working across—do not start in the middle of the fire.
To prevent vehicle fires: never start a vehicle with a fuel leak (repair it and absorb spills); shut off the engine when refueling; touch the fuel nozzle to the filler pipe to ground it and prevent static sparks; don't smoke near fueling areas; check tire pressure often (soft tires build heat); and ensure brakes are fully released when moving (dragging brakes ignite hub grease).
Once the fire is out: replace the safety pin and return the extinguisher to its storage compartment. Note on the post-trip inspection that the extinguisher has been used, and have it recharged immediately or replaced.
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