8.3 Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks
Key Takeaways
- The inspection starts with full tanks, cycles the pressure down with the tractor protection check, builds it back up with the supply circuit check, and then tests for leaks with the system full again.
- Step 2 set-up: engine off with the key in the run position, red trailer air supply valve pushed in, yellow park control pulled out, and both air lines to the trailer disconnected.
- In Step 2, the red valve should pop out at 40–60 PSI (276–414 kPa) or higher and the low-air warning should come on by 60 PSI; then apply and hold the foot or hand valve, and no air should leak from the open service line.
- Step 3: push in the yellow park control, pump the foot valve, and confirm it pops out at 20–45 PSI (138–311 kPa); then reconnect both air lines.
- Each of these eight checks is worth 5% on the Air Brake Practical Test form, so together they are worth 40% of the practical score.
Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks
Quick Answer: Alberta's trip air brake inspection has six steps, set out in the Commercial Driver's Guide and the Air Brake Practical Test form. It starts with full tanks. Step 1: chock the wheels and inspect the components visually. Step 2 (tractor protection): turn the key to run with the engine off, push the red button in and pull the yellow button out, then disconnect both trailer lines. The red button must pop out at 40–60 PSI or higher, and the low-air warning must come on by 60 PSI. Then apply and hold the brakes: no air should leak from the open service line. Step 3 (park control): push the yellow button in and pump the pedal; it must pop out at 20–45 PSI. Then reconnect both lines.
Why the steps come in this order
The instructor manual explains the logic of the sequence:
- Start with the air tanks full.
- Cycle the pressure down with the tractor protection system check (Step 2), and keep lowering it with the park control check (Step 3).
- Build it back up with the supply circuit check (Step 4), which needs low pressure to start the build-up test.
- Test for leaks with the system full again (Step 5).
Each step sets up the next. Doing them out of order wastes time you do not have on a 15-minute practical test.
Step 1: Set-up and visual inspection
- Chock the wheels with the vehicle on level ground.
- Visually inspect the air brake components (Section 8.2).
Step 2: Tractor protection system
| Action | What you should see | Why |
|---|---|---|
| Leave the engine off with the key in the run position | Gauges and warnings powered | The compressor must not refill the tanks during the check |
| Push the red trailer air supply valve in; the yellow park control should be pulled out | Trailer charged; tractor spring brakes applied | The yellow button holds the unit on its springs, and the red button charges the supply line |
| Disconnect both air lines to the trailer | Air escapes from the open supply coupler, and tractor pressure falls | Simulates a breakaway |
| Watch the red button | It should pop out at 40–60 PSI (276–414 kPa) or higher | Proves the trailer supply valve protects the tractor's air |
| Watch the low-air warning | It should come on by 60 PSI (414 kPa) | This can also be checked in the supply circuit section |
| Apply and hold the foot valve or the hand valve | No air leaks from the open trailer service line | Proves the tractor protection valve has closed the service line |
How to disconnect a live line. Place your hand between the glad hand couplers and your face so the escaping air cannot blow debris into your face, and wear appropriate PPE.
If the red button trips at once. On some vehicles, the trailer supply valve trips immediately after the lines are separated. In that case, return to the cab and firmly apply the foot valve to confirm that the tractor protection valve has closed the trailer service line.
The same check at roadside. In a Level 1 CVSA inspection, officers remove both lines, wait for the red button to trip, and ask the driver to apply the service brake. The service line must not leak. A tractor protection system that fails puts the vehicle out of service.
Step 3: Park control valve
The park control valve is not part of the tractor protection system, but it is efficient to check it here while the pressure is already coming down.
| Action | What you should see |
|---|---|
| Push the yellow park control valve in | Air goes to the spring brakes |
| Pump the foot valve | Pressure drops further |
| Watch the yellow button | It should pop out at 20–45 PSI (138–311 kPa) |
| Reconnect both air lines to the trailer | Inspect the glad hands before connecting, connect them properly, confirm the connection, and check that the lines are suspended so they will not catch as the trailer turns |
Notes from the course:
- The low-air warning can also be confirmed here: with the yellow button pushed in, reduce pressure until it pops out.
- The yellow button sometimes pops out below 20 PSI, even on new vehicles. Make sure it pops out to its default park position before the tanks are empty. A park control that does not default to park when the air is depleted puts the vehicle out of service.
How Steps 1–3 are scored on the practical test
On the Air Brake Practical Test form, each of these items carries 5%:
| Section | Item on the form | Weight |
|---|---|---|
| Tractor protection | Push the red button; the yellow should be pulled | 5% |
| Tractor protection | Disconnect both air lines to the trailer | 5% |
| Tractor protection | Confirm the low-air warning comes on by 60 PSI (414 kPa) | 5% |
| Tractor protection | Confirm the red button pops out at 40–60 PSI (276–414 kPa) or higher | 5% |
| Tractor protection | Apply and hold the foot or hand valve; no air leaks from the open service line | 5% |
| Park control valve | Push the yellow button and pump the foot valve | 5% |
| Park control valve | Confirm the yellow button pops out at 20–45 PSI (138–311 kPa) | 5% |
| Park control valve | Reconnect both air lines to the trailer | 5% |
These eight items total 40% of the form. Because 25% or more in errors fails the test, missing five of them would fail you on its own.
How should the vehicle be set up at the start of the tractor protection system check (Step 2)?
With both trailer lines disconnected and the red valve popped out, the driver applies and holds the foot valve. What result shows the tractor protection valve is working?
On some tractors the red trailer air supply valve trips as soon as the trailer lines are separated. What does the course say to do?