Each time you drive a vehicle with air brakes, you must make sure that there are no defects and that the vehicle complies with the Ontario Highway Traffic Act and regulations. This chapter describes the inspection procedure you must follow. It also prepares you for the Ministry of Transportation air brake endorsement practical examination.

Secure the vehicle before you start

To conduct the inspection, park your vehicle on level ground to prevent it from rolling. Apply the spring brakes and put wheel chocks or blocks in place to secure the vehicle.

Important: Never leave the driver’s seat or get under a vehicle unless the vehicle’s spring brakes are applied or the wheels are securely blocked.

Testing low air-pressure warning devices

To test the low air-pressure warning device, pressure in the system must be reduced to the point where the device activates or 380 kPa (55 psi), whichever is higher. The pressure at which the warning device deactivates during a rise in pressure is not necessarily the same point at which it activates during a drop in pressure. Begin the test with the pressure above 621 kPa (90 psi) and the engine stopped or running. With the ignition switch turned on, reduce pressure by repeatedly pressing and releasing the brake pedal.

Observe the air-pressure gauges and note when the device activates. Most warning devices will activate above 414 kPa (60 psi). If the warning device fails to activate at a minimum of 380 kPa (55 psi), the low air-pressure warning device is defective.

Step-by-step procedure

  1. Ensure air brake system pressure is above 621 kPa (90 psi).
  2. The engine may be stopped or left running but the ignition must be on.
  3. Observe the primary and secondary air tank pressure gauges.
  4. Press and release the brake pedal repeatedly to lower air pressure.
  5. Watch and listen for the low air-pressure warning device to activate.
  6. When the device activates, note the air pressure displayed by the gauges.

Pass

The vehicle passes the test if the device activates when pressure is at or above 380 kPa (55 psi) on both primary and secondary air tank gauges.

Fail

The vehicle fails the test if the device does not activate or activates when pressure is below 380 kPa (55 psi) on either the primary or secondary air pressure gauge.

Important: The Ontario Highway Traffic Act and regulations prohibit the operation of a vehicle with a defective low air pressure warning device.

Testing air-pressure build-up time

The air compressor must be capable of meeting the demand of the air brake system and restoring pressure to the normal range quickly. This is confirmed by testing whether air pressure rises to a specified level within a specified time.

To test the air-pressure build-up time, reduce the system pressure to below 552 kPa (80 psi). If the vehicle has a trailer attached, ensure the trailer supply valve is closed (pulled out). With the engine idling between 600 and 900 rpm, observe the time required for the air pressure to rise from 587 kPa (85 psi) to 690 kPa (100 psi).

If the time required to build up air pressure exceeds two minutes, the air brake system is defective.

Step-by-step procedure

  1. Lower air brake system pressure to below 552 kPa (80 psi).
  2. Run the engine at 600 to 900 rpm.
  3. Observe the primary and secondary air-tank pressure gauges.
  4. Note the time when pressure reaches the start value of 587 kPa (85 psi).
  5. Note the time when pressure reaches the end value of 690 kPa (100 psi).

Pass

Vehicle passes the test if pressure build-up time is equal to or less than two minutes.

Fail

Vehicle fails the test if pressure build-up time is greater than two minutes.

Important: The Ontario Highway Traffic Act and regulations prohibit operation of a vehicle that fails to build air pressure within the prescribed time.

Testing air-compressor governor settings

Air brake systems must operate with air pressure within a prescribed range. The system’s pressure range is controlled by the air-compressor governor settings, which determine when the air compressor will cut-out and cut-in. Drivers can perform a test to determine the settings and establish the normal operating pressure range for a particular vehicle.

The model year of a vehicle generally affects the governor pressure setting. Air brake system operating pressure ranges have increased over the past 20 years. Older systems may operate with lower pressure settings.

Air pressure gauges stop climbing with compressor cut-out. When a vehicle uses an air dryer, its exhaust cycle also indicates that the compressor has reached the cut-out setting. Observe the primary and secondary air-tank gauges to confirm when the pressure stops climbing and when the cut-out setting has been reached.

The cut-in pressure setting is normally 138 to 173 kPa (20 to 25 psi) below the cut-out pressure setting. Compressor cut-in causes a change in the sound of the engine and can be observed when the air-tank gauges begin to show an increase in pressure.

Cut-out and cut-in pressures should remain within the range specified by the vehicle manufacturer, and any change in these pressures, should be reported. Actual cut-out pressure must never be higher than 1000 kPa (145 psi). Actual cut-in pressure must never be less than 552 kPa (80 psi). The air brake system is defective when cut-out pressure is above 1000 kPa (145 psi) or cut-in pressure is below 552 kPa (80 psi).

Step-by-step procedure

  1. Properly secure the vehicle and release the spring brakes.
  2. Observe the primary and secondary air-tank pressure gauges.
  3. Run the engine until air brake system pressure reaches its maximum level and note the cut-out pressure setting.
  4. Press and release the brake pedal several times to lower the system pressure and note the cut-in pressure setting.

Pass

The vehicle passes the test when the cut-in and cut-out pressure settings are within the range specified by the vehicle manufacturer, cut-out pressure is below 1000 kPa (145 psi), and cut-in pressure is above 552 kPa (80 psi).

Fail

The vehicle fails the test when actual cut-out pressure is above 1000 kPa (145 psi) or actual cut-in pressure is below 552 kPa (80 psi).

Important: The Ontario Highway Traffic Act and regulations prohibit operation of a vehicle with air-compressor governor settings outside the prescribed limits.

Testing system air-loss rate

Drivers must be alert for air brake system leaks and pressure loss in the air tanks when brakes are not being used. These conditions indicate air loss in the air brake system. For safety, drivers should test the air-loss rate of the vehicle’s brake system.

To test the air-loss rate of the brake system, release the spring brakes, establish normal air pressure and shut off the engine. Hold the brake pedal in the fully applied position and observe the air-pressure readings for one minute.

The pressure will drop noticeably when the brakes are first applied, but must not continue to drop at a rate greater than specified in the chart below. The amount of pressure drop that takes place when brakes are first applied is not considered when performing the air- loss rate test. The air brake system is defective when air loss exceeds the specified values.

Step-by-step procedure

  1. Properly secure the vehicle and release the spring brakes.
  2. Ensure that the air brake system is within its normal operating pressure range. Shut off the engine.
  3. Press and hold the brake pedal in the fully applied position.
  4. Note the pressure indicated on the primary and secondary air tank gauges.
  5. Note the change in pressure over one minute.

Pass

The vehicle passes the test when the drop in pressure is equal to or less than the value specified for the vehicle.

Fail

The vehicle fails the test when the drop in pressure exceeds the value specified for the vehicle.

Important: The Ontario Highway Traffic Act and regulations prohibit operation of a vehicle with an excessive air loss rate.

Air Loss Rates

Type of vehicleMaximum allowable air loss
Straight truck, tractor or bus21 kPa (3 psi) per minute
Tractor and trailer28 kPa (4 psi) per minute
Tractor and two or more trailers41 kPa (6 psi) per minute

Testing a tractor (towing vehicle) protection valve

The tractor protection valve on a towing vehicle ensures that an air- loss problem in the trailer does not result in loss of air from the towing vehicle.

To test the tractor protection valve, the trailer supply valve must be closed (pulled out), the trailer service line must be disconnected, and the service brakes applied. No air should be exhausting from the trailer service line. If air exhausts from the service line, the tractor protection valve is defective.

Step-by-step procedure

  1. Ensure that the air brake system is within its normal operating pressure range.
  2. Ensure the trailer supply valve is closed (pulled out).
  3. Disconnect the trailer service air line coupler from either the trailer or the dead-end coupler and place it where it can be observed.
  4. Press and hold the brake pedal. (If there is a concern that the vehicle has no anti-compounding valve, ensure the vehicle is secure and release the spring brakes before applying service brakes.)
  5. Observe whether air is exhausting from the trailer service-line coupler.

Pass

The vehicle passes the test if air does not exhaust from the trailer service line.

Fail

The vehicle fails the test if air exhausts from the trailer service line.

Important: The Ontario Highway Traffic Act and regulations prohibit the operation of a vehicle with a defective tractor protection valve.

Testing the automatic application of the trailer spring brakes

A trailer’s spring brakes must automatically apply whenever the trailer is disconnected from the towing vehicle. To test this, open (push in) the trailer supply valve to fully charge the trailer. Then pull out the trailer supply valve to close it. The trailer spring brakes should apply. Disconnecting the trailer air-supply line also activates this function, but closing the trailer supply valve is the recommended testing method. Brake application may be confirmed by gently applying engine power to move the vehicle forward or backward.

If the trailer spring brakes fail to apply automatically when the trailer supply valve is closed, the trailer brakes are defective.

Step-by-step procedure

  1. Ensure the trailer supply valve is open (pushed in) and the trailer is fully charged.
  2. Ensure the air brake system is within its normal operating pressure range.
  3. Pull out the trailer supply valve to close it.
  4. Observe the trailer for application of the trailer’s spring brakes.
  5. If necessary, confirm brake application by attempting to gently move the vehicle forward or backward.

Pass

The vehicle passes the test if the trailer spring brakes apply automatically.

Fail

The vehicle fails the test if the trailer spring brakes do not apply.

Important: The Ontario Highway Traffic Act and regulations prohibit the operation of a vehicle with defective trailer spring brakes.

Testing the spring (parking and emergency) brakes

A vehicle’s spring brakes must be capable of holding the vehicle in place. If the test is being conducted on a towing vehicle and trailer, select the vehicle on which to perform this test. It is preferable that this test be performed on the spring brakes of the towing vehicle. However, when a towing vehicle is pulling a trailer, it may not be possible to test its spring brakes separately. This can be done only on towing vehicle systems where the trailer can be supplied air while the parking brakes of the towing vehicle are released. Spring brakes can be tested by gently applying engine power in a low gear while the brakes are applied. The vehicle may rock slightly but the wheels should not turn during the test.

Failure of the spring brakes to hold the vehicle stationary indicates defective spring brakes.

Step-by-step procedure

  1. Apply the spring brakes on the vehicle to be tested and remove wheel chocks.
  2. Gently apply engine power in a low gear.
  3. Observe the vehicle’s response. The vehicle may rock and shake and the wheels may move slightly, but there should be no significant movement of the vehicle.

Pass

The vehicle passes the test if the spring brakes hold the vehicle in place.

Fail

The vehicle fails the test if the spring brakes do not hold the vehicle in place.

Important: The Ontario Highway Traffic Act and regulations prohibit the operation of a vehicle with defective spring brakes.

Testing the air-tank drain valves

Air tanks must be drained regularly, and the discharge observed for abnormalities. Some moisture may be discharged from the supply tank. A much smaller amount of moisture may be discharged from the remaining air tanks. A significant quantity of moisture being discharged from the supply tank - even when the tank is drained on a regular basis - may be normal. Discharge of a significant quantity of moisture from the remaining air tanks is not normal and should be reported.

While a small amount of oil may be found in the supply tank, any visible quantity of oil should be reported or repaired. When oil is found in any other air tank, there is risk of air brake system contamination, and the condition must be reported.

When there is a sudden increase in the amount of moisture or oil drained from any tank, the condition must be reported and repaired. Any malfunctioning drain valve must be repaired.

The supply tank should always be drained first to prevent accumulated moisture in the supply tank passing farther into the system. Drivers must know the location of all air tanks and drains.

It is important to note that the body design and suspension of some vehicles may limit safe access to the air tanks and drains unless the vehicle is supported on a hoist, or is over a pit or ramp.

Step-by-step procedure

  1. Ensure that the air brake system is within its normal operating pressure range.
  2. Locate and drain the supply tank until the valve discharges only clean air.
  3. Locate and drain the remaining air tanks.
  4. Watch the discharge from each air-tank and ensure that all air-tank drain valves function properly.

Pass

The vehicle passes the test when each drain valve functions properly.

Fail

The vehicle fails the test when any drain valve fails to function properly.

Important: The Ontario Highway Traffic Act and regulations prohibit the operation of a vehicle with defective air tank drain valves.

Key points to remember

  • To test low air-pressure warning devices, activate the device by reducing air-pressure. If the device fails to activate or activates below 380 kPa (55 psi), the low air-pressure warning device is defective.
  • To test air-pressure build-up time, lower air-pressure to less than 552 kPa (80 psi), run the engine at 600 to 900 rpm and observe the time it takes for pressure to rise from 587 to 690 kPa (85 to 100 psi). If the air-pressure build-up time is greater than two minutes, the air brake system is defective.
  • Test air-compressor governor settings by observing the compressor cut-in and cut-out settings. If cut-out pressure is greater than 1000 kPa (145 psi) or less than 690 kPa (100 psi), or cut-in pressure is less than 552 kPa (80 psi), the air brake system is defective.
  • Test air-loss rate by checking for audible air leaks and then observe the air pressure gauges with full air-pressure, the engine stopped, spring brakes released and service brakes applied. If the pressure drop in one minute exceeds the value specified for the vehicle, the air brake system is defective.
  • Test the tractor protection valve by listening for air exhausting from the trailer service line with the trailer supply valve closed, the trailer service line disconnected and the service brakes applied. If air is exhausting from the trailer service line, the tractor protection valve is defective.
  • Test the automatic application of the trailer spring brakes with the trailer supply valve open (pushed in) and the system fully charged. Pull out the trailer supply valve to close it and listen for the trailer spring brakes to apply automatically. If the brakes do not apply automatically, the trailer spring brakes are defective.
  • To test the effectiveness of the spring brakes, gently apply engine power with the spring brakes applied. If the spring brakes fail to hold the vehicle stationary, the spring brakes are defective.
  • To test the air-tank drain valves, drain each air tank while observing the discharge. If any drain valve fails to work properly or has an abnormal discharge of moisture or oil, the air brake system is defective.