6.2 Pre-Trip Practical Tests

6.2 Pre-Trip Practical Tests

While the static checks assess system integrity and safety fail-safes when the vehicle is stationary and the engine is off, the pre-trip practical tests evaluate the active components of the air brake system. These tests are conducted with the engine running and under slow-moving conditions. They verify the performance of the air compressor, the operational range of the governor, and the actual physical holding power of both the parking and service brakes.

Air Compressor Build-up Rate Test

The air compressor is the heart of the air brake system. It must be capable of generating enough air pressure to replenish the reservoirs during normal driving, where frequent braking can deplete the air supply. The compressor build-up rate test measures the compressor's volumetric efficiency under load.

Step-by-Step Procedure:

1. With the vehicle parked and the wheels blocked, fan the service brakes until the system pressure drops below 50 psi (345 kPa).
2. Start the engine.
3. Increase and maintain the engine speed to a fast idle, typically between 1000 and 1200 rpm (revolutions per minute).
4. Watch the primary and secondary pressure gauges.
5. Using a stopwatch, start timing the instant the pressure needle passes 50 psi (345 kPa).
6. Stop timing the instant the pressure needle reaches 90 psi (620 kPa).
7. Under British Columbia regulations, the compressor must build the air pressure from 50 to 90 psi within 3 minutes (180 seconds).

Mechanical Diagnostic:

If the build-up rate takes longer than 3 minutes, the compressor is not operating at full efficiency. Common causes of a slow build-up rate include:

• A slipping or worn air compressor drive belt.
• A clogged or restricted air compressor intake filter, which limits the volume of air the compressor can draw in.
• Carbon build-up in the compressor discharge line, which restricts air flow to the reservoirs.
• Worn compressor piston rings or valves, allowing air to bypass back into the crankcase.
• Significant air leaks in the supply line or the air dryer assembly.

A failed build-up test indicates a system that may not be able to recover pressure during stop-and-go driving or prolonged brake applications, creating a severe safety risk.

Governor Cut-in and Cut-out Test

The governor controls the operating range of the air compressor. It directs the compressor to pump air (load) or stop pumping air (unload) to maintain safe system pressures. The governor must be tested to ensure it operates within the legal limits.

Testing Governor Cut-out:

1. Run the engine and monitor the primary and secondary pressure gauges.
2. Allow the pressure to rise until the governor cuts out.
3. You will hear a distinct exhaust or "sneeze" of air from the air dryer's purge valve, and the pressure gauges will stop rising. Note that the purge valve exhaust is the definitive physical indicator of cut-out.
4. Note the pressure. Under ICBC regulations, the governor must cut out between 120 and 135 psi (828 to 931 kPa).
5. If the governor cut-out exceeds 135 psi, it can over-pressurize the system, potentially damaging lines, hoses, or valves. If it cuts out below 120 psi, the system will not have sufficient reserve pressure.

Testing Governor Cut-in:

1. With the engine idling, slowly press and release (fan) the service brake pedal to reduce the air pressure.
2. Watch the gauges.
3. Note the pressure when the compressor cuts back in (begins pumping air again), which is indicated by the pressure needles starting to rise.
4. The governor must cut in at least at 80 psi (552 kPa). Typically, cut-in occurs around 100 to 105 psi (about 20-25 psi below the cut-out pressure).
5. If the cut-in pressure is below 80 psi, the air pressure could drop to a level where the low-air warning activates or braking efficiency is reduced before the compressor starts rebuilding pressure.

The Tug Tests

The tug tests are physical checks designed to confirm that the mechanical brakes can hold the vehicle's weight against engine torque. These tests verify the adjustment, lining condition, and mechanical operation of the parking brakes and trailer brakes before driving.

Parking Brake Tug Test:

1. Ensure the vehicle's parking brakes are fully applied (the yellow dashboard button is pulled out).
2. If testing a tractor-trailer, push the red trailer supply valve in to release the trailer brakes, ensuring you are testing only the tractor's parking brakes.
3. Place the transmission in a low gear.
4. Gently release the clutch (or apply light throttle for automatic transmissions) to tug against the parking brakes.
5. The vehicle must hold and not move. If the vehicle moves, the tractor parking brakes are defective, out of adjustment, or have worn/contaminated linings.

Trailer Parking Brake Tug Test (Combination Vehicles):

1. Pull the red trailer supply valve out to apply the trailer's emergency/parking brakes.
2. Push the yellow tractor parking brake button in to release the tractor's parking brakes. This isolates the trailer brakes.
3. Put the vehicle in a low gear.
4. Gently release the clutch (or apply light throttle) to tug against the trailer brakes.
5. The trailer brakes must hold the entire combination. If the tractor is able to pull the trailer forward, the trailer emergency brakes are not holding. This could indicate a leak in the trailer supply line, out-of-adjustment trailer brakes, or defective trailer spring brake chambers.

Service Brake Test (Rolling Test):

1. Release all parking brakes by pushing in both the yellow and red dashboard buttons.
2. Drive the vehicle forward slowly at approximately 3 to 5 km/h (walking pace).
3. Depress the service brake pedal firmly.
4. The vehicle must stop smoothly and immediately.
5. The steering wheel should not pull to the left or right.
6. If the steering wheel pulls or the vehicle veers, it indicates unequal braking force. This can be caused by brakes on one side being out of adjustment, oil or grease contamination on the brake linings, a seized slack adjuster, or a leaking brake chamber.

Practical Tests Summary Table