7 step air brake test

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What is the 7 Step Air Brake Test?

A commercial driver must perform a pre-trip inspection before taking a truck trailer on any public road. The most important part of the pre-trip inspection is the 7 step air brake test.

Please note that this article is not an actual air brake test, this is just for practice.

Always do the 7 step air brake test in order. Initially, it is difficult to memorize the 7 steps air brake test, but the word “LAGASS” makes it easier to remember the steps in the 7 step air brake test. The abbribation ofpsi “LAGGASS” is

  •  L for Low Air Warning Device.
  •  A for Air Pressure Buildup.
  • G for Governor Cut Out.
  • G for Governor Cut In.
  • A for Air Loss Rate.
  • S for Spring Brake Test.
  • S for Service Brake Test.

Tractor Trailer Pre-Trip Inspection

How to do a 7 step air brake test?

Before starting the air brake test, park the vehicle on a flat surface, apply spring brakes and wheel chokes. Start testing with the low air warning device.

1. The low air warning device test

There is an indicator on the vehicle’s dashboard that warns the driver when the air pressure drops below the required level. This device can be an audible warning, an indicator or both.

How to test the low air warning device?

  • Make sure the air pressure is above 90psi.
  • Turn the imagination switch on or keep the engine running.
  • Pump down the brake pedal and keep an eye on the dashboard(where the visible indications appears, mostly on the dashboard, in some vehicles in the gauges) and air pressure gauges.
  • A visible and or audible signal must activate on or before 55psi.

If the low air warning device does not activate, it is a major defect.

2. The air pressure build-up test.

This test evaluates the working of the air compressor. When travelling on the road, the air pressure drops with the frequent use of brakes. But, the air compressor keeps filling up the air tanks as per instructions from the governor to meet the required air pressure. The compressor must fill the required air within a time constraint, called air pressure build-up. Air pressure build-up depends on your province, in Ontario it is 85psi – 100psi in 2 minutes engine running in between 600 to 900 rpm.

How to test air pressure buildup?

  • Make sure the engine is running in between 600rpm to 900rpm.
  • Pump down the brake paddle and lower the air pressure to 85psi.
  • Now keep watching the needles in the air pressure monitoring gauges.
  • The needles must start moving and go from 85psi to 100psi within 2 minutes. In other words, the air pressure must build up from 85psi to 100psi within 2 minutes.

3. Governor cut-out test

What is the governor cut-out test?

Governor cut out, governor cut in
The governor in the air brake system

When the required air pressure in the air brake system is reached, the governor shuts off the air compressor, this is called a governor cut out. we hear a “thesis.”  sound when the governor cuts out. This noise comes from the governor cut out and the discharge from the air dryer. The minimum governor cut-out is 100psi and the maximum is 145psi.

Note down the governor’s cut-out reading.

How to test the governor cut-out?

  • Keep the engine running at 600 to 900rpm.
  • Watch the needles moving and open the driver window.
  • listen to the (“thisssss”) sound. This is the governor cut-out.
  • The minimum governor cut-out is 100psi and the maximum is 145psi.
air dryer working
Air dryer purging down the debris

Why is the governor cut-out test important?

If the governor does not cut, the air pressure in the brake system will continue increasing, which can damage the brake system. But if the governor fails and the air pressure continues to rise,the safety valve(located on air tanks) opens up at 150psi and saves the air brake system.

4. Governor cut-in test

What is the governor cut-in Test?

The moment the compressor starts throwing air into air tanks is called governor cut in. The governor must cut in at 20psi to 25psi below the governor cut-out.
The governor cut-in and the cut-out is the normal operating range(NOR) of an air brake system.

How to test the governor cut-in?

  • Pump down the brake paddle and lower the air pressure 20psi to 25psi from the cut-out reading.
  • Now, look at the air pressure monitoring gauges. The governor is cutting in if the needles start moving, which means the air compressor started throwing air into the air tanks.
  • If the needles do not start moving, pump down the peddle again and lower the air pressure by 5psi and watch the needles again.
  • Repeat the above until the air pressure reaches 80psi.

The minimum governor cut in is 80psi. Means governor must cut in at 80psi.

5. Air loss rate test.

What is the air loss rate test?

The air loss rate test is a measurement of air leakage from the air brake system.

How to test the air loss rate?

  • Make sure the vehicle is on the level surface and the air pressure is within the normal operating range(cut-out and cut-in).
  • Apply the wheel chocks.
  • Turn the engine off.
  • Release the spring brakes.
  • Apply and hold the service brakes for a minute.
  • Open the window and listen for the air leaks.
  • The air brake system in a tractor-trailer must not lose more than 4psi in a minute after an initial loss.

The air loss rate limits are 3psi (straight truck, bus), 4psi (tractor-trailer) and 6psi (tractor and two trailers) in a minute.

6. Service brake test.

What is a service brake test?

Debris under the brake paddle, truck brake paddle, tractor brake paddle, cleaning brake paddles, mats, brake paddle covor
Debris under the brake paddle, tractor brake paddle

The service brake test is a test of a vehicle’s stopping ability. Your brake paddle is a service brake. Check the brake peddle for rust, debris and the cleanness of the area. The debris under the brake paddle, loose objects on the floor can create dangerous situations.

How to check the service brakes?

  • Apply the spring breaks.
  • Remove the wheel chocks.
  • Now release the spring brakes and put the vehicle in drive.
  • Let the vehicle roll and apply the service brakes.

The service brakes must hold the vehicle in parallel.

7. Spring brake test

What is the spring brake test?

The spring brakes test is the test of the parking brakes. There is a push/pull yellow knob on the dashboard for the tractor parking brakes.

How to test the spring brakes?

  • Put the vehicle into the transmission and let it roll for a yard.
  • Apply the spring brakes pulling the yellow knob.

The spring brakes must hold the vehicle in parallel and smooth.

Why is the spring brake test Important?

Spring brakes are parking brakes, if they are not working we won’t be able to park the vehicle.

The next steps in air brake testing are…
1. Draining the air tanks.
2. Pushrod measurement.
You can find both steps in the link below.

How to adjust air brakes

Download in cab 7 step air brake test pdf here

CDL 7 Step Air Brake Test video(please read the air pressure buildup time 2 mins in the video)


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