How To Assess The Braking System For Possible Faults
Your examiner may ask you what you would do if you suspect that your vehicle has a fault in its braking system. To react appropriately to this question it is necessary to understand how an air-braking system works.
Vehicles equipped with air brakes use compressed air to apply the brakes – this means that their braking system is very different to smaller vehicles that have hydraulic braking systems using brake fluid.
A compressor attached to the engine recharges a reservoir – this compressor is working whenever the engine is running.
A gauge on the dashboard confirms the pressure of the air in the reservoir – check where the gauges are, there will be one each for two reservoirs. Sometimes the gauge is provided as part of a display screen instead of using a separate instrument.
If the pressure in the reservoir is low, an alarm sounds – if air is low it is possible that braking could be impaired, never ignore the alarm.
When the driver presses the brake pedal, air pressure from the reservoir applies the brakes – each application of the brakes uses some of the air in the reservoir, air leaks and faults with the compressor could cause the system to run out of air.
When the driver releases the brake pedal this air is released to the atmosphere – you can hear this release of gas as large vehicles come to a halt and the driver releases the brakes.
If air is leaking from the system between the compressor and the brake pedal the gauge on the dashboard will slowly fall, even though the brakes have not been used – if you come to a vehicle after leaving it overnight and find that the reservoir is empty there may well be a leak. Reservoirs also have a drain valve to allow moisture to be drained. If your reservoirs are empty it may be because they have been drained rather than because of an air leak.
A hiss of escaping air may be heard when the brake pedal is not being pressed – if this is the case then immediate repair is required. If compressed air is being lost between the reservoir and the brake pedal then the total braking capacity of the system is reduced. You should listen for this type of leak while conducting your regular daily walk around check.
If air is leaking from the system between the brake pedal and the brakes – this is very serious as some of the pressure that should reach the brakes is being lost. When the vehicle is un-laden there may well be enough pressure for the system to behave as normal, but when fully laden there may not be enough pressure to brake efficiently and safely.
If air is leaking from the system between the brake pedal and the brakes …
A hiss of escaping air may be heard when the brake pedal is being pressed – it may be possible to hear this from inside the cab. To ensure that there is no leak however, the brake should be depressed while the driver walks around the vehicle. Tell the examiner that this is what you want to do and ask for assistance.
Your examiner may ask you how you would check that your compressor is working properly. To do this you need to discharge the contents of the reservoirs and recharge them.
With the engine off, pump the brake to discharge the reservoir – remember that air is lost from the system every time the brake is used and released. For this reason repeated use of the brake pedal with the engine switched off will empty the reservoir.
The gauge will fall further with each use of the brake until the low-pressure warning sounds. At this point you can stop pumping the brakes.
Run the engine at a fast idle to recharge the reservoir – once the engine is started the compressor will start to recharge the reservoirs. Running the engine at a fast idle will charge the system a little faster…
Do this until the alarm stops and the gauge shows normal pressure – continue running the engine, don’t stop as soon as the alarm stops but rather wait until the needles are well into the normal operational range.
To summarise;
To check for air leaks between the compressor and brake pedal;
- With the engine off, listen for leaks while walking around the vehicle
To check for air leaks between the brake pedal and the brakes;
- With the engine off and the brake pedal depressed, listen for leaks – get help to check around the vehicle for leaks.
- Visually inspect components and the airlines if possible.