Air Brakes Basics

 

Air brakes are the standard braking system used in heavy commercial vehicles. They use compressed air to operate the brakes, making them more effective for large trucks and buses.

 

How Air Brakes Work: Air brakes use compressed air stored in tanks to apply pressure to the brake pads and stop the vehicle. When you press the brake pedal, air is released into the brake chambers, applying pressure to the brakes.

 

Components of Air Brake Systems

 

There are three main parts of the air brake system:

 

1. Service Brake – Applies the brakes when you press the brake pedal.

 

2. Parking Brake – Uses air pressure to keep the brakes locked when parked.

 

3. Emergency Brake – Applies when there is a loss of air pressure.

 

Key Air Brake Components:

 

• Air Compressor: Generates compressed air for the brake system.

 

• Air Tanks (Reservoirs): Store the compressed air.

 

• Brake Chambers: Convert the air pressure into mechanical force to apply the brakes.

 

• Brake Drums or Discs: The components that come into contact with the brake shoes to stop the wheels.

 

• Slack Adjusters: Adjust the brakes to maintain the correct brake pad distance from the drums or discs.

 

Air Brake System Operation

 

• The air brake system operates at normal pressure levels between 100-125 PSI. The air compressor will cut in and cut out to maintain proper pressure in the system.

 

• Governor: Controls the air compressor, ensuring the correct air pressure is maintained.

 

Applying the Brakes:

 

• When the brake pedal is pressed, air is released from the tanks into the brake chambers. This air applies pressure to the brakes, stopping the vehicle.

 

• Modulating Valve: Helps control the amount of air applied to the brakes, allowing for gradual stopping instead of abrupt braking.

 

Air Brake Safety Systems

 

Low Air Pressure Warning:

 

• A warning light or buzzer will activate if air pressure drops below 60 PSI. If this happens, you should pull over immediately and inspect the system.

 

Spring Brakes:

 

• If the air pressure falls too low (below 20-45 PSI), the spring brakes will automatically engage. These are mechanical brakes that ensure the vehicle can still stop in case of air pressure loss.

 

Brake Lag:

 

• There is a slight delay (about ½ second) between when you press the brake pedal and when the brakes engage due to the time it takes for air to flow through the system. This is known as brake lag.

 

Pre-trip Air Brake Inspection:

 

Before driving, you must inspect the air brake system to ensure it’s functioning correctly.

 

1. Check Air Pressure: Ensure that the air pressure is between 100-125 PSI before driving.

 

2. Test Low Air Pressure Warning: Release air from the system and check that the low air pressure warning activates before it drops below 60 PSI.

 

3. Test Spring Brakes: Continue releasing air and ensure the spring brakes activate when the air pressure drops below 20-45 PSI.

 

4. Check Slack Adjusters: Make sure they are adjusted properly. Too much slack means the brakes won’t apply correctly.

 

Brake Fade:

 

• Brake fade occurs when the brakes overheat due to excessive use, like driving downhill without using proper braking techniques. This can reduce the brakes’ effectiveness.

 

Proper Braking Techniques

 

Controlled Braking:

 

• Apply the brakes in a steady, firm manner without locking the wheels. This allows you to maintain control while stopping.

 

Stab Braking:

 

• If your vehicle starts to skid, release the brakes briefly and then reapply them, allowing the wheels to regain traction.

 

Downhill Braking:

 

• Use your brakes intermittently when driving downhill. Keep your speed under control by applying the brakes for short bursts, allowing them to cool in between.