8.4 Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring

Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring

Quick Answer: Step 4 (supply circuit): at a fast idle around 1200 RPM, pressure must build from 50 to 90 PSI within 3 minutes, and the low-air light should go out by 60 PSI. The governor should cut out at 120–135 PSI and cut in 20–25 PSI below cut-out. Step 5 (leaks): with the engine off and the spring brakes released, hold a full service application for 2 minutes. The maximum loss is 4 PSI for the power unit plus 2 PSI per trailer, measured after the system stabilizes. On the practical test you then make full service applications to maintain the automatic slack adjusters and confirm the adjustment on one axle. Step 6 is the brake response test with the foot valve and the hand valve.

Step 4: Supply circuit

Recognizing the events. At cut-out, the gauges stop rising and the air dryer purges. At cut-in, the gauges stop falling and start to rise as the compressor loads again. Record the pressure at each point. The cut-in must be judged against your cut-out reading: a cut-out of 125 PSI calls for a cut-in of 100–105 PSI, while a cut-out of 130 PSI calls for 105–110 PSI.

If a check fails. A slow build-up is a minor defect to record and report (Section 8.1). Cut-out or cut-in outside the ranges, or a warning that does not behave correctly, needs repair. A low-air warning that fails is a major defect.

Step 5: Air system leaks

Worked allowances over the 2-minute hold:

This one procedure checks the whole air brake system for leaks. If a leak is found, you have to track it down by isolating the subsystems. The only line not filled during the test is the control line from the hand valve to the double check valve, on vehicles with hand valves. Be careful not to mix up standards: some other jurisdictions state their applied leak limits as a rate per minute. Alberta's check is 4 PSI plus 2 PSI per trailer over a 2-minute hold.

Brake adjustment (practical test)

After the air system checks, the practical test covers brake adjustment on the tractor-trailer:

1. Restart the engine to keep the air pressure in the normal range (above 100 PSI / 690 kPa).
2. With the spring brakes released, make full service applications (two in the course), pressing the pedal firmly to the floor with about one-second intervals to allow for brake lag. This maintains the adjustment of the automatic slack adjusters. Do not fan the pedal.
3. Confirm the brake adjustment on both brakes of one axle by pulling the slack adjusters (Section 6.3). The rear trailer axle is often the easiest.

In the full trip inspection every brake is inspected. In the practical test, only one axle needs to be checked.

Step 6: Service brake response

The Commercial Driver's Guide finishes the trip air brake inspection with:

1. Remove the wheel chocks.
2. Release the spring park brakes.
3. Do a brake response test using the foot valve.
4. Do a brake response test using the trailer hand valve.

The vehicle should respond promptly and stop without pulling to one side.

How the practical test is scored

The Air Brake Practical Test form (DPLS0003) has five groups of tasks:

Rules:

• Time limit: 15 minutes. If the required tasks are not finished in time, the test is discontinued.
• One-on-one, with no other students present, and no notes or list. You practise with the form, but you cannot use it in the test.
• The instructor may prompt the next group of tasks, for example "Show me how you would maintain the automatic slack adjusters", but may not coach individual steps.
• 80% is needed to pass, so errors totalling 25% or more fail.
• A retest must be on a different day and is scored on the same form. Only one practical re-assessment is allowed before additional training is required.

Scoring examples

The brake adjustment group carries a quarter of the marks, which reflects how much weight the course puts on adjustment.