Study guide
5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits
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
5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits
Key Takeaways Explain This Topic AI
A supply circuit failure shows as gauges that stop recovering after dropping below governor cut-in; all brakes still work, but only on the air stored in the service tanks.
In a primary circuit failure, the spring brakes stay released on secondary air, and a service application brakes the steering axle and the trailer.
Many trucks have a tractor spring brake (inversion) valve that, in a primary failure, lets controlled air out of the drive-axle spring brakes so they apply in step with the pedal.
A secondary circuit failure is the only tractor air failure in which a service circuit's brakes do not apply: the steering axle brakes will not work.
A park circuit failure drains both service circuits; make a steady service application and expect the spring brakes to apply gradually as pressure falls to about 55–60 PSI.
Last updated: September 2026
Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits
Quick Answer: Each tractor failure has its own signs and its own braking result. Supply circuit failure: the gauges do not recover after falling below cut-in, and the brakes work only on the stored air. Primary failure: the steering axle and trailer brake. On many trucks an inversion (spring brake) valve also applies the drive axles through their springs. Secondary failure: the steering axle brakes do not apply , which makes it the only tractor failure that disables a service circuit's brakes. Park circuit failure: both service circuits lose air, and the spring brakes gradually apply around 55–60 PSI . In every case, use a steady brake application and stop safely.
How to think about failures
The Alberta course pairs each failure with two questions: what does the failure look like, and what happens when you then make a service application? Answer those two questions for each circuit and you can handle any failure question on the test. Three facts from earlier chapters drive every answer:
Check valves protect each service tank from a failure upstream (Section 2.3).
The park circuit and the trailer supply draw from the higher-pressure service tank (Section 3.1).
The steering axle is on the secondary circuit, and the drive axles are on the primary circuit.
1. Supply circuit failure
What fails: anything from a plugged or collapsed line to a ruptured supply tank. Air from the compressor no longer reaches the service tanks.
How you recognize it: the check valves protect the service tanks, so nothing may seem wrong at first. The warning sign is that the gauges do not replenish when pressure falls below the governor cut-in point. Every application lowers the pressure, and it does not come back.
What still works: all of the brakes work properly , but only on the air already stored in the service tanks . That reserve is at least the 12 full applications that power-unit tanks are sized for, less whatever you have used.
What to do: do not keep driving and hoping. Plan a controlled stop while you still have plenty of pressure, use steady applications rather than repeated ones, and get the fault repaired.
2. Primary circuit failure
What fails: a line, chamber, or tank in the primary (green) circuit, which normally feeds the drive axles.
How you recognize it: the primary gauge drops , and the low-air warning comes on when it falls below 60 PSI.
Spring brakes: they stay released , because the park circuit now draws its air from the secondary tank.
On a service application: the steering axle and trailer brakes apply . The drive axles have no service air. Many trucks, however, have a tractor spring brake valve , sometimes called an inversion valve , which is drawn to the right of the foot valve in the course schematics. The secondary service signal controls it, and it lets a controlled amount of air out of the drive-axle spring brakes . The springs then apply the drive-axle foundation brakes in proportion to your pedal application, and they release again when you let off. More air is used to re-release the springs each time, so the reserve still runs down.
What to do: a steady brake application to a safe stop. Do not pump.
3. Secondary circuit failure
What fails: a line, chamber, or tank in the secondary (red) circuit, which normally feeds the steering axle.
How you recognize it: the secondary gauge drops and the low-air warning comes on.
Spring brakes: they stay released , on air from the primary tank.
On a service application: the drive axles and trailer brake normally, but the steering axle brakes do not apply . The course points out that this is the only tractor air system failure that stops a service circuit's brakes from working during a service application . In a primary failure the drive axles can still be braked through the inversion valve, but nothing replaces the steering-axle service brakes.
What to do: allow more stopping distance, make a steady application, and stop safely.
4. Park circuit failure
What fails: a line or chamber in the orange park circuit. The park circuit is fed from both service tanks, through the double check valve that selects the higher one, so a leak there draws air out of both service circuits .
How you recognize it: both gauges fall and the low-air warning comes on.
What happens next: as pressure falls, the spring brakes begin to apply. They start to take hold at about 55–60 PSI and are firmly applied by about 45–50 PSI.
What to do: the course's advice is to apply a steady service application and expect the spring brakes to begin to gradually apply as pressure drops to about 55–60 PSI.
Will the wheels lock? Students often ask this. A spring park application is only about half as strong as a full service application . Whether the wheels lock depends on the vehicle's weight , how sharply the brakes apply (a shock application or a gradual one), and the available traction . Once the spring brakes apply, you have no control of the vehicle if it starts to slide. ABS operates only on the service circuits and will not help.
Summary table
Failure Gauges Spring brakes Brakes on a service application Driver action
Supply circuit Stop recovering below cut-in Released All brakes, on stored air only Controlled stop while pressure remains
Primary Primary drops; warning on Released (secondary air) Steering axle + trailer; drive axles by springs if an inversion valve is fitted Steady application; do not pump
Secondary Secondary drops; warning on Released (primary air) Drive axles + trailer; no steering axle Steady application; more distance
Park circuit Both drop; warning on Apply gradually from about 55–60 PSI Service brakes while air lasts; then springs Steady service application; expect the springs to take hold
General rules for any air loss
Do not pump the brakes. Each release exhausts chamber air, so pumping drains the reserve quickly.
Use steady, controlled applications , and downshift to use engine braking where it is safe.
Stop in a safe place and secure the vehicle before the spring brakes take over by themselves.
Do not drive on. An activated low-air warning or an inoperative service or parking brake is a major defect.