Study guide
5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks
Alberta Air Brake Q Endorsement
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1 Chapter 1: Alberta Q Endorsement Program and Air Brake Fundamentals
1.1 Alberta Q Endorsement Path, Course Requirements, and Knowledge Test Logistics 1.2 Licensing Scope, Farm Vehicle Exceptions, and the Technician Route 1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours
2 Chapter 2: The Supply Circuit: Compressor, Governor, Tanks, and Air Dryers
2.1 Air Compressors, Drive Systems, and Governor Operation 2.2 Supply and Service Tanks, Contamination, and Draining 2.3 Air Dryers, Air Brake Antifreeze, Safety Valves, and Check Valves
3 Chapter 3: Service Circuits, Control Valves, Gauges, and Low-Air Warnings
3.1 Dual Air Systems: Primary and Secondary Service Circuits 3.2 Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves 3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response
4 Chapter 4: The Park Circuit and Tractor/Trailer Circuits
4.1 Spring Parking Brakes, the Park Control Valve, Compounding, and Caging 4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands 4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers
5 Chapter 5: Air System Failures and Emergency Braking
5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits 5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks
6 Chapter 6: Foundation Brakes and Brake Adjustment
6.1 Foundation Brakes: S-Cam Drum, Disc, and Wedge Brakes 6.2 Brake Stroke, Adjustment Limits, and Stroke Indicators 6.3 Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters
7 Chapter 7: Braking Fundamentals, Driving Techniques, and Brake Management Systems
7.1 Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade 7.2 Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking 7.3 Antilock Brakes, Automatic Traction Control, and Stability Control
8 Chapter 8: Vehicle Inspections and the Air Brake Practical Test
8.1 Trip Inspection Law, Major and Minor Defects, and Roadside Enforcement 8.2 Inspection Set-Up and Recognizing Air Brake Defects 8.3 Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks 8.4 Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring
5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks
Key Takeaways Explain This Topic AI
A trailer service line failure can reduce a 63,500 kg Super-B's braking capacity to that of a loaded 22,500 kg tandem straight truck.
If there is room to stop gradually after a trailer service line failure, use a steady application, release at about 70 PSI (483 kPa) to let pressure rebuild, and keep downshifting.
If the trailer service brakes are not working and the vehicle must stop quickly, pull the red trailer air supply valve to apply the trailer's emergency brakes.
A trailer supply (emergency) line rupture applies the trailer emergency brakes through the tractor protection system; a sliding trailer usually swings right on a crowned road and to the outside in a turn.
When you push in the red valve after coupling, the gauges should drop as the trailer fills; if they do not, the trailer tanks are already full or the lines are reversed.
Last updated: September 2026
Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks
Quick Answer: A trailer service line failure can leave a heavy combination with only a fraction of its braking. If there is room, make a steady application , release at about 70 PSI (483 kPa) so pressure can rebuild, and keep downshifting . If you must stop quickly , pull the red trailer air supply valve to apply the trailer's emergency brakes. A supply (emergency) line rupture applies the trailer brakes through the tractor protection system. If the trailer then loses traction it swings , usually to the right on a crowned road. Reversed lines keep a spring brake trailer's brakes from releasing. A trailer tank failure shows as unusually low readings on both gauges.
1. Trailer service line failure
What fails: the service (blue) line, which carries the application signal to the trailer relays.
Why it is so dangerous: the service line sits unpressurized until you brake, so nothing seems wrong until you step on the pedal. Then the signal escapes through the break and the trailer brakes do little or nothing. The Alberta course gives an example: a Super-B combination weighing 63,500 kg (139,994 lb) has its braking capacity cut to that of a tandem straight truck loaded to 22,500 kg (49,604 lb) . Braking distances increase dramatically.
Response when there is enough road ahead for a gradual stop:
Make a steady service brake application .
Watch the air gauges and release the application at about 70 PSI (483 kPa) . The pressure rebuilds when the service brakes are released .
Keep downshifting. Higher engine speed lets the compressor rebuild pressure faster and makes an auxiliary retarder more effective.
Repeat as needed until you are stopped safely.
Response when you must stop quickly: drivers have little time to think in an emergency. If the trailer service brakes are not working and the vehicle must be stopped quickly, pull the trailer air supply valve (red button). This exhausts the supply line, and the trailer's emergency or spring brakes apply.
Using the dash controls for emergency braking. The dash park control is rarely used for emergency braking. It would be used where the service brakes have been lost and a quick stop is needed, such as a service line failure or a trailer service tank failure. The drawback: on a tractor fitted with a bobtail drive-axle relay , the drive-axle service braking is reduced by about half once the trailer supply is exhausted. You will have to press the pedal harder to get the same braking from the tractor.
2. Trailer supply (emergency) line rupture
What fails: the supply (red) line tears or disconnects, from a breakaway, a snagged line, or a coupler that was not secured.
What happens: all control of the trailer is lost. The tractor protection system should activate on the towing vehicle, and the trailer emergency brakes apply .
Will the trailer lock and swing? That depends on the weight on the axles, how rapidly the brakes apply, and the traction. ABS has no effect on an emergency application. If the trailer does lose traction, it will swing :
typically to the right , because of the pavement crown , and
in a turn, to the outside of the turn , because of centrifugal force.
A supply line failure is very dangerous for other road users. That is why the course stresses air line security during coupling, pre-trip inspections, and en-route inspections . If there is any doubt about the security of the glad hand couplers, repair it before driving .
3. Reversed trailer lines
- Trailer type Effect of reversed (crossed) lines
- Polarized glad hands in good shape They cannot be reversed
- Spring brake trailer The trailer's park brakes will not release , because the supply port gets no constant air
Older air park brake trailer Can be a real problem. If the trailer tank is empty , there are no service or park brakes . While driving, repeated service applications slowly fill the trailer tank through the wrong line, and the air park brakes start to drag
How to catch it: when you push in the red trailer air supply valve after coupling, watch the tractor gauges drop as the trailer fills. That drop is expected. A tridem trailer with empty tanks and air-ride suspension can take a lot of air. If the gauges do not drop, either the trailer tanks are already full or the lines are reversed.
4. Trailer air tank failure
How you recognize it: an unusually low reading on both air pressure gauges . The tractor keeps trying to fill a trailer tank that will not hold air.
Response: the same as for a trailer service line failure . Use a steady application with release at about 70 PSI and downshifting if you have room, or pull the red valve if you must stop quickly.
Keep the trailer charged during inspections
During pre-trip and en-route inspections, push in the red trailer air supply valve with the yellow park control pulled out . The trailer stays charged, the tractor's springs hold the unit, and any trailer air leaks can be heard and fixed before the equipment goes into service.
Summary
Failure Sign Best response
Trailer service line Weak trailer braking and falling pressure during applications Room to stop: steady application, release at about 70 PSI, downshift. Must stop quickly: pull the red valve
Trailer supply (emergency) line Trailer emergency brakes apply; tractor protection activates Steer and brake the tractor under control; expect trailer swing if it slides
Reversed lines Spring brakes will not release, or gauges do not drop when charging Reconnect correctly; check polarization