Tractor-Trailer Air Brake Systems +6British Columbia · CDL · English

A trailer system has foundation brakes, air chambers, reservoirs and control valves, but no air compressor. It relies on the tractor for compressed air (via the supply line) and control signals (via the control line). Air lines connect with quick-coupling glad hands, colour-coded blue for control and red for supply.

• ✓The only major component not found on a trailer air system is an air compressor
• ✓The supply line (also called emergency line) fills the trailer reservoirs; the control line (also called service line) carries the control signal
• ✓Glad hands are colour-coded: blue for the control line, red for the supply line

All trailer systems must have an emergency stopping system that fully applies the trailer brakes if the trailer separates. Trailers without spring parking brakes use a relay emergency valve which senses supply line pressure loss and applies full reservoir pressure to the brakes (dynamiting). Spring-brake-equipped trailers use spring force to dynamite.

• ✓All trailer systems must have an emergency stopping system that fully applies the trailer brakes if the trailer separates from the tractor
• ✓A relay emergency valve applies full trailer reservoir pressure (dynamites the brakes) if it senses supply line pressure loss
• ✓Trailers with spring parking brakes use spring force to dynamite the brakes

Trailer brakes are dynamited when the supply line breaks, when glad hands are disconnected, or when the driver closes the trailer supply valve. Motor vehicle safety standards require the dynamited brakes to remain applied for a minimum of 15 minutes. Small leaks in non-spring-brake systems can allow a parked trailer to roll away.

• ✓Trailer brakes dynamite when the supply line breaks, glad hands disconnect, or the trailer supply valve is closed
• ✓Dynamited trailer brakes must remain applied for a minimum of 15 minutes per safety standards
• ✓Always block the wheels of a parked trailer so it cannot roll

Trailers with spring parking brakes use a relay valve for service brakes and a trailer spring brake valve to fill the reservoir and control the spring parking brakes. Two types exist: one fills the reservoir before releasing the spring brakes; the other releases the spring brakes first then fills the reservoir. Always perform a tug test after coupling.

• ✓The trailer spring brake valve fills the reservoir and controls the spring parking brakes
• ✓Always perform a tug test after coupling the tractor to the trailer
• ✓Before a tug test, visually check with a flashlight that the fifth wheel locking jaws are closed and locked

If the trailer separates, the connecting air lines break and tractor air can be lost. The tractor protection system (a trailer air supply valve in the dash and a tractor protection valve behind the cab) automatically shuts off air loss, preserving pressure for a safe stop. Some shut off immediately, some allow drop to as low as 20 p.s.i.

• ✓The tractor protection system consists of a trailer air supply valve in the dash and a tractor protection valve behind the cab
• ✓It automatically shuts off tractor air loss in a breakaway to preserve pressure for a safe stop
• ✓A peace officer may place a vehicle out of service if the system allows tractor pressure to drop below 20 p.s.i. (138 kPa) without the trailer air supply valve automatically closing

The dash-mounted trailer air supply valve (usually an octagon-shaped red button) directs air pressure to the trailer supply line. It senses supply line pressure and closes automatically in a breakaway. When no trailer is connected it stays in the closed position for bobtail driving.

• ✓The trailer air supply valve is usually an octagon-shaped red button that controls air to the trailer
• ✓A peace officer may place a trailer out of service if the trailer brakes don't apply when the trailer air supply valve is closed
• ✓When no trailer is connected, the trailer air supply valve remains closed for bobtail driving

The hand valve allows the trailer brakes to be applied independently of the tractor, useful when coupling. It should not be used in normal or emergency service braking; always use the foot valve. Most hand valves are spring-loaded and return to released. Never use the hand valve for parking.

• ✓The hand valve applies only the trailer brakes independently of the tractor
• ✓Do not use the hand valve for normal or emergency service braking; always use the foot valve
• ✓Do not use the hand valve for parking; apply the tractor parking brakes, close the trailer supply valve, and block the trailer wheels

A dual air tractor towing a trailer adds a trailer air supply valve, tractor protection valve, hand control valve, and a pair of two-way check valves. The two-way check valves send a control signal to the trailer whether the foot valve or hand valve is applied. The parking brake control valve is usually interlocked with the trailer supply valve.

• ✓Components added to tow a trailer include the trailer air supply valve, tractor protection valve, hand control valve, and two two-way check valves
• ✓The parking brake control valve (yellow button) is usually interlocked with the trailer supply valve (red button) so both tractor and trailer parking brakes apply together
• ✓Two-way check valves send a control signal to the trailer whether the foot valve or hand valve is applied

With a primary air system failure on the tractor, the low-air warning alerts the driver. A foot valve application applies the front (secondary) brakes and passes air through both two-way check valves to signal trailer brakes. Automatic shutoff of trailer air supply occurs when the highest-pressure reservoir drops to between 20 and 45 p.s.i.

• ✓With primary failure, a foot valve application applies the tractor front brakes and signals the trailer brakes
• ✓Automatic shutoff of trailer air supply occurs when pressure is lowered to between 20 and 45 p.s.i. (138 and 310 kPa)
• ✓With blended air, shutoff won't occur until the highest-pressure reservoir drops into the 20-45 p.s.i. range
• ✓The automatic shutoff should be checked during pre-trip; if it fails, the vehicle must be placed out of service

In a breakaway, the sudden air loss through the broken supply line causes the trailer air supply valve to shut off automatically. The tractor protection valve closes the passage to the trailer control line so no application air is wasted. If only the control line separates, nothing happens until the trailer brakes are applied, then protection activates.

• ✓Sudden air loss in a breakaway causes the trailer air supply valve to shut off automatically
• ✓The tractor protection valve closes the passage to the trailer control line to prevent wasting air
• ✓If only the control line separates, protection activates when the trailer brakes are applied

When not in use, glad hands should be fastened to dead-end (dummy) couplers or to each other to prevent dirt and debris from entering the air lines, and secured to prevent chafing or bouncing.

• ✓When not in use, glad hands should be fastened to dead-end couplers or to each other
• ✓Secure glad hands to prevent lines from chafing against components or bouncing off the vehicle

If the supply and service (control) line connections are reversed during coupling, the spring parking brakes won't release even with the dashboard control, and the trailer service brakes won't function. Avoid this by taking care during coupling.

• ✓Reversed line connections cause spring parking brakes not to release and service brakes not to function
• ✓Take care when coupling to avoid reversing the supply and service line connections

The two-way check valve directs the highest application pressure from either the hand valve or the foot valve to the trailer brakes, allowing independent trailer brake application. It is identical in construction to the one used in spring parking brake installations.

• ✓The two-way check valve directs the highest application pressure (from hand valve or foot valve) to the trailer brakes
• ✓It allows the trailer brakes to be applied independently

Driving a tractor without a trailer is called bobtailing. With little weight over the rear axles, rear brakes lock easily. A bobtail proportioning system reduces drive axle braking pressure by up to 75 per cent when bobtailing while giving full pressure to steering axle brakes, improving control and shortening stopping distance. Higher pedal pressure is required.

• ✓Driving a tractor without a trailer attached is called bobtailing
• ✓A bobtail proportioning system reduces drive axle braking pressure by up to 75 per cent when bobtailing
• ✓Because steering axle brakes do most of the braking when bobtailing, higher than normal pedal pressure is required

Brakes convert kinetic energy to heat and can only handle a limited amount of heat before losing effectiveness (brake fade). Overheated brakes can fail, especially on long downgrades.

• ✓Brakes have a limited capacity to handle heat before fading
• ✓Excessive braking heat causes brake fade and possible failure
• ✓Manage heat buildup, especially on downgrades

Braking on icy roads and roads covered with water requires gentle, controlled braking to maintain traction. Water on roadways can cause hydroplaning and reduce braking effectiveness.

• ✓Brake gently on icy roads to maintain traction
• ✓Water on roadways reduces braking effectiveness and can cause hydroplaning
• ✓Reduce speed in wet or icy conditions

Braking a combination unit requires understanding how the tractor and trailer brakes work together to maintain stability and prevent jackknifing or trailer swing.

• ✓Combination units require coordinated braking of tractor and trailer
• ✓Improper braking can cause jackknifing or trailer swing

Heavy vehicles require far more energy and distance to stop than to start moving. Understanding stopping distance and time is critical for safe operation of loaded commercial vehicles.

• ✓Stopping distance and stopping time increase dramatically with speed and weight
• ✓Heavy vehicles need much greater distance to stop than lighter vehicles

Brakes convert motion into heat, and there is a limit to how much heat brakes can handle before they fade. Excessive braking on long descents can overheat brakes and cause loss of braking.

• ✓Brakes have a limit to how much heat they can handle before fading
• ✓Overusing service brakes on downgrades causes brake fade

On long downgrades drivers should select an appropriate lower gear and use controlled braking to prevent overheating. Different techniques apply on highways versus in town.

• ✓Select a lower gear before descending a long downgrade
• ✓Use controlled/intermittent braking to avoid overheating brakes on downgrades

Retarders help slow a heavy vehicle without using the service brakes, reducing brake wear and heat buildup. They must be used with caution on slippery surfaces.

• ✓Retarders reduce reliance on service brakes and help control speed on downgrades
• ✓Use retarders cautiously on icy or wet roads to avoid loss of traction

Runaway lanes provide an escape for vehicles that lose braking on steep grades. On icy roads braking must be gentle to maintain traction.

• ✓Use a runaway lane if you lose braking control on a steep downgrade
• ✓Brake gently on icy roads to avoid wheel lockup and skidding

ABS prevents wheels from locking during hard braking, allowing the driver to maintain steering control. Automatic traction control helps prevent wheel spin during acceleration.

• ✓ABS prevents wheel lockup and helps maintain steering control during hard braking
• ✓Automatic traction control reduces wheel spin during acceleration on slippery surfaces

To prevent a runaway vehicle: set the parking brake in the tractor; place the transmission in lowest forward gear if facing uphill, reverse gear if facing downhill, or park for automatics. With main and auxiliary transmissions, place both in gear; two-speed axles must be in low range. Apply parking brakes on both tractor and trailer. Stop the engine, lock the ignition and remove the key.

• ✓Set the parking brake in the tractor and apply parking brakes on both tractor and trailer
• ✓Place transmission in lowest forward gear facing uphill, reverse facing downhill
• ✓Two-speed axle must be in low range and both main/auxiliary transmissions in gear
• ✓Stop engine, lock ignition and remove the key

Most trailers with air brakes have spring brakes. If your trailer has no spring brakes, apply the trailer brakes and block the wheels because air pressure may bleed down and release the trailer brakes. Never use a trailer hand valve to hold an unattended unit — air may drain and brakes may release. Block wheels of any large vehicle parked on even a slight grade. Spring brakes are effective only when brakes are correctly adjusted.

• ✓If a trailer has no spring brakes, apply trailer brakes and block the wheels
• ✓Never use a trailer hand valve to hold an unattended unit
• ✓Block wheels of any large vehicle parked on even a slight grade
• ✓Spring brakes are effective only when brakes are correctly adjusted

On an uphill with a curb (single-unit), turn wheels left toward the centre of the road. On an uphill with no curb (single-unit), turn wheels right toward the edge. For a tractor-trailer combination uphill (with or without curb), always turn wheels left toward the centre. On any downgrade, always turn wheels right toward the edge of the roadway.

• ✓Single-unit uphill with curb: turn wheels left toward centre of road
• ✓Single-unit uphill no curb: turn wheels right toward edge of roadway
• ✓Tractor-trailer uphill (curb or no curb): always turn wheels left toward centre
• ✓Any vehicle on downgrade: turn wheels right toward edge of roadway

Division 35 of the Motor Vehicle Act Regulations (MVAR) specifies requirements for transporting logs/poles, covers for loose loads, front-end structures and bus luggage. B.C. has adopted the North American Standard on cargo securement, which applies to all cargo types with specific requirements for certain commodities. Both Division 35 and the Standard apply to all vehicles carrying cargo on highways regardless of gross vehicle weight. Unless the Standard specifies otherwise, general and specific requirements both apply.

• ✓Division 35 of MVAR and the North American Standard both apply to cargo securement
• ✓Rules apply to all vehicles carrying cargo regardless of gross vehicle weight
• ✓General and specific requirements both apply unless otherwise specified
• ✓Do not operate a vehicle unless cargo is secured properly

The cargo securement system must suit the cargo's size, shape, strength and characteristics, and may include the vehicle structure, blocking/bracing equipment and securing devices such as tiedowns. The system must withstand 0.8g deceleration forward, 0.5g deceleration rearward, 0.5g acceleration in either sideways direction, and a downward force equal to at least 20% of cargo weight. Load on each component must not exceed its working load limit.

• ✓Must withstand 0.8g forward deceleration
• ✓Must withstand 0.5g rearward deceleration and 0.5g sideways in either direction
• ✓Must withstand downward force of at least 20% of cargo weight
• ✓Load on each component must not exceed its working load limit

Cargo securement system components must be in good working order — not damaged, cracked, cut or weakened — and fit for their intended purpose. Components in poor condition must not be used.

• ✓Components must not be damaged, cracked, cut or weakened
• ✓Components must be fit for the purpose for which they are used

If a load can't be reduced and is larger or heavier than regulations allow, you may purchase permits for overwidth, overweight, overlength or overheight loads. Apply through the Commercial Vehicle Permit Line. You must provide registration/insurance papers, load specifics, route/destination details and travel time. Overload permit applicants must identify weight on each axle.

• ✓You must have a permit before you take an oversize vehicle on the road
• ✓Provide registration and insurance papers for both tow vehicle and trailer(s), load specifics, route details and travel time
• ✓For overload permits you must identify the amount of weight on each axle
• ✓To move oversize/overweight loads on city or municipal roads you may need a permit from local government

Overloading makes steering, braking and speed control more difficult. A vehicle is overloaded when it exceeds GVW allowed under regulations, exceeds its licensed GVW, or exceeds maximum allowable weight on any single or group of axles. Overload permits are only issued when loads can't be reduced or the vehicle isn't licensed to its allowable weight.

• ✓A vehicle is overloaded when it exceeds regulation GVW, its licensed GVW, or the maximum weight on any single or group of axles
• ✓Overload permits are issued only when loads can't be reduced or the vehicle isn't licensed to allowable weight
• ✓Drive slowly on upgrades and be careful not to lose speed control going downhill when overloaded

If your vehicle is wider, longer or higher than regulations allow, you must have an oversize permit before operating on any highway, whether loaded or not. Permits for loaded vehicles are only given when the load can't be reduced. Permits are not normally given for loads wider than 4.4 m. Oversize extremities must be marked with red flags at least 30 cm by 30 cm, with amber lights front and red rear for night travel.

• ✓You must have an oversize permit before operating on any highway whether loaded or not
• ✓Permits are not normally given for loads or vehicles wider than 4.4 m
• ✓Extremities of oversize vehicles must be marked with red flags at least 30 cm by 30 cm
• ✓For night travel, amber lights are required to the front and red lights to the rear on extremities

When you receive a permit you will be given a list of requirements to fulfill, which may include pilot cars, flags, signs, lights, permitted time of day, days of the week you may travel, and the route you must use.

• ✓Permit requirements may include pilot cars, flags, signs, lights, restricted times/days and specific routes
• ✓You must fulfill all listed requirements when operating under a permit

A pilot car may be required as a condition of an oversize or overload permit to warn other drivers about the larger-than-normal vehicle. More than one pilot car may be required depending on route and load. A pilot car must travel between 100 and 500 m either behind or in front of the oversize vehicle. When required, a D (over dimension) sign must be carried by the pilot car.

• ✓A pilot car warns other drivers about a larger-than-normal vehicle approaching
• ✓A pilot car must travel between 100 and 500 m either behind or in front of the oversize vehicle
• ✓When required, a D (for over dimension) sign must be carried by the pilot car
• ✓More than one pilot car may be required depending on route and load

You must display the appropriate signs whenever you transport an oversize load. Different signs warn of wide load, long load or oversize load being carried.

• ✓Display the appropriate signs whenever transporting an oversize load
• ✓Signs indicate whether a wide load, long load or oversize load is being carried

Air pressure alone is unreliable and illegal for parking because any leak or failure could cause loss of pressure and a rollaway. Regulations require the parking force be maintained by mechanical means, unaffected by loss of air pressure. The two types are spring parking brakes and safety actuators.

• ✓Parking force must be maintained by mechanical means and be unaffected by loss of air pressure
• ✓Using only air brakes for parking is illegal and unsafe

Most spring parking brakes consist of an additional chamber attached behind the service brake chamber containing a powerful coil spring (1,500 to 2,000 pounds of force). The spring applies the brakes through the service chamber pushrod. Spring parking brakes are mounted on rear axles only, not steering axles. They are applied and held by mechanical spring pressure, not air.

• ✓Spring parking brakes are mounted on rear axles only, not on steering axles
• ✓The coil spring exerts between 1,500 and 2,000 pounds of force and holds brakes applied by mechanical pressure, not air
• ✓Spring brake assemblies should only be serviced by qualified personnel due to extreme spring pressure

Spring brakes are normally applied and released using the parking brake control valve (usually a yellow button) on the dashboard. Pushing in supplies air to release; pulling out exhausts air to apply. If air pressure falls below approximately 60 p.s.i. the springs may drag, and at 20 to 45 p.s.i. (138 to 310 kPa) they may automatically fully apply.

• ✓Pull the parking brake control valve out to apply (exhaust air), push in to release (supply air)
• ✓At 20 to 45 p.s.i. (138 to 310 kPa) spring brakes may automatically apply; they may begin dragging below approximately 60 p.s.i.

When spring parking brakes are applied (up to 2,000 lbs force) and a heavy service brake application is made, the air force (up to 3,000 lbs) is added to the spring force for a total of up to 5,000 lbs. This compounding can damage slack adjusters, s-cams, mounting bolts, rollers, shoes and drums. Always release spring brakes before heavy service applications.

• ✓Always release spring parking brakes before making a heavy service brake application
• ✓Light applications under 30 to 40 p.s.i. (207 to 276 kPa) to hold the vehicle while releasing/applying spring brakes are not harmful

Spring parking brake chambers can be manually released or 'caged' using a release bolt, nut and washer. This should only be done by mechanics during repairs or in emergencies (e.g. to tow a vehicle when all air is lost). Once caged, there is no parking brake force at that wheel.

• ✓Always block the wheels before caging spring parking brakes
• ✓Once a spring brake chamber is caged, there is no parking brake force at that wheel

A two-way check valve supplies the parking brake dash control with air from the reservoir with the highest pressure — called blended air. It has two inlet ports and one delivery port with a free-floating shuttle that moves away from the higher pressure inlet. This ensures spring brakes won't automatically apply if only one reservoir loses air.

• ✓The two-way check valve draws air from the reservoir with the highest pressure (blended air)
• ✓This ensures spring parking brakes will not automatically apply if there is total loss in only one reservoir

An optional spring brake modulator senses loss of pressure in the primary system, and when the driver applies the service brakes, exhausts air from the spring parking brakes in direct proportion to the brake application. This lets the driver control spring force to help the front brakes make a controlled stop.

• ✓Senses loss of primary system pressure and modulates spring brake application proportional to foot valve application
• ✓All vehicles must meet Canadian Motor Vehicle Safety Standards for emergency stopping even with a partially failed system

Used on many buses and highway coaches, safety actuators use a one-way locking mechanism instead of a coil spring. They have two diaphragms (service and parking) and a separate parking air reservoir. Pulling the dash control applies air to the parking diaphragm and engages the locking mechanism, holding the pushrod in the applied position even if all air is lost.

• ✓Safety actuator parking brakes cannot be compounded because spring force is not used
• ✓They will not apply automatically even if service reservoir pressure drops to zero — only loss of parking reservoir pressure causes automatic application
• ✓If more than 4 p.s.i. (27.6 kPa) is lost from the parking reservoir, a heavy service brake application must be made to release them

Foundation brakes are the most important part of any braking system; if not properly adjusted, even the best system can't stop the vehicle safely. Brake failures and runaways are almost never caused by air system failure — usually lack of maintenance or failure to check brakes daily. The driver is ultimately responsible before the vehicle moves.

• ✓Daily pre- and post-trip inspections including checking and adjusting brakes are mandatory
• ✓The driver may be held responsible if brakes are incorrectly adjusted or not working
• ✓One person is ultimately responsible — that person is the driver

Federal and provincial laws require checking manual and automatic slack adjusters daily during pre-trip inspection. Drivers must also check brakes before driving down steep grades posted with regulatory signs. You may be fined as much as $2,000 for operating a vehicle with defective brakes, including excessive brake chamber pushrod stroke.

• ✓Federal and provincial laws require daily checks of slack adjusters during pre-trip inspection
• ✓You must check brakes before driving down steep grades posted with regulatory signs
• ✓You may be fined as much as $2,000 for operating with defective brakes