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How to Use Air Brakes on a Truck? Facts You Should Know

How to Use Air Brakes on a Truck? Facts You Should Know

Truck air brakes use compressed air as transmission media for the driver’s force on the brake pedal to the wheels.

How to Use Air Brakes on a Truck? When you apply the brakes, compressed air from the storage tank reaches the brake chambers via brake valves and pushes the brake shoes against the drum. Friction between the two causes the truck to stop or slow down according to the applied force.

How to Use Air Brakes on a Truck?

Air brakes are popular among drivers of heavy vehicles like trucks, trailers, and buses due to their fail-safe operation.

These brakes always work to stop or slow down the moving truck, ensuring the driver’s safety and other traffic on the road.

This feature makes them stand out from hydraulic brakes that do not work due to hydraulic fluid loss.

However, their operation needs skills and adequate training due to varying air pressures, transmission lag, and more than one air circuit in the truck.

Therefore, it is safe and recommended practice by the government for the drivers to learn about the working principle and various parts of the air brake system.

Parts of air brake system

They have a complex structure and various parts to ensure their efficient operation by reducing air leakage.

We will discuss the main components one by one to give you an idea before narrating the working principle.

Air compressor

It is a mechanical assembly that operates on engine power via a belt or gears to compress air for the brakes. It has several types, mainly the rotary type you will find in trucks.

When it rotates, it takes in fresh air from the inlet side and compresses it to high pressure in air storage tanks for use in brakes whenever required.

A pressure governor controls the operation of the air compressor. Its purpose is to regulate the pneumatic circuits to prevent too high or too low-pressure conditions.

Typical working pressure ranges are 95-125 psi (pound per square inch), but they can vary according to your truck.

However, a pressure below 85 psi is too low and above 150psi is too high for the air brake system. 

A pressure gauge in front of the driver keeps him updated about the system’s health and proper air pressure.

If too high or too low pressures, the driver should safely stop the vehicle and inspect the system for any problem.

Brake pedal and piston

A brake pedal is a mechanical device located on the floor of your pickup that you can operate with your right foot.

The cast-iron structure with the footpad makes it durable for the driver to deliver his force to the brakes. 

On the brake end, pneumatic lines end up in brake chambers where they operate an air-operated piston or diaphragm.

This piston acts as a transducer to convert the potential energy of compressed air into mechanical energy.  

Pneumatic pipes

All components of the air braking system are connected through rubber hose pipes to supply the compressed air.

Valves and a governor control the airflow in these lines according to the operational requirements.

These pipes should be securely fixed in their places and free from any kinks to avoid leakage or obstruction in the airflow.

The pneumatic lines for service brakes should have blue color while emergency brakes’ pipes have a red color. This color scheme will be helpful while troubleshooting any fault in the brakes.  

Brake shoes

They are the standard part of electric, hydraulic and pneumatic brakes. Their function is to provide a frictional force on the brake side during the braking operation.

A small portion of the drum and shoe surface erodes during every stop, causing them to wear out. 

Brake drum

The brake drum is a cylindrical hollow structure fixed with the wheel hub with bolts’ help, and it rotates along with the wheel.

The rest of the brake assembly is static and encapsulated by the drum. During braking operation, friction between the shoes and drum stops the vehicle.

Due to rubbing, high amounts of heat produce. Therefore it should have durable material to withstand such heavy friction and high temperatures.

It is a standard part of hydraulic and electric brakes as well.

Compressed air storage tank

Air storage tanks are a vital part of the pneumatic braking system. 3-4 or more air cylinders located under the truck body have mild steel as their construction material.

Compressed air from the compressor enters the first tank. This air contains moisture and lubricant oil from the compressor.

That moisture and oil mixture needs to be drained regularly through a drain valve on the bottom of the tank.

After the first tank, 2 other tanks are forming two circuits to ensure air supply in case of one loop’s failure.

The air in these cylinders is relatively dry and is in ready to function form.

The storage reservoirs’ size should be enough to support the brakes in case of the air compressor’s failure.

Sometimes, the air dryer between the compressor and supply tank removes a significant portion of air’s moisture content. 

Another part of the air tanks can be a heater or antifreeze arrangement to avoid ice formation and reduce air humidity.

It is essential to enhance brakes’ service life as moisture in the air can cause corrosion and rapid aging of the parts. 


Valves in the air brake system control the airflow in the whole circuit. Most of the valves are relay-operated and work on compressed air.

Some common valve types are foot valve or triple valve, inversion valve, safety valve, treadle valve, pressure protection valve, dual parking control valve, spring brake valve, and quick release valve.

The triple valve serves the three purposes of connecting the compressor to the reservoir tank, releasing air to the atmosphere, and supplying air to the brake’s service lines.

Inversion valve allows controlled release of air from the spring chamber for smooth braking instead of springs’ abrupt departure.

The Treadle valve controls the air supply to the brake chambers and opens and closes according to pressure variation on the valve’s inlet and outlet side. 

Pressure protection valves preserve the air pressure to the service brakes by isolating it from the auxiliary system in case of leakage.

A quick-release valve allows releasing brakes quicker when the driver depresses the brake pedal by removing the lines’ air pressure.

A safety valve automatically operates to release air pressure if the air pressure exceeds certain limits to preserve the system from damage.

Slack Adjuster

It is a mechanical assembly to make adjustments to the brake shoes. It is accessible on the backside of the backing plate.

It connects the push rod with the S-shaped cam that pushes the brake shoes against the drum during brakes’ operation. It can be self-adjusting or manually adjusting by rotation of a nut.


It is a small mechanical part that works as an actuator to transfer the force of compressed air to the brake’s mechanical assembly via a slack adjuster.

Its length is appropriately adjusted to ensure the smooth operation of the air brake.

Brake chambers

It is a hollow chamber fitted with check valves to allow airflow and a diaphragm that moves with air pressure.

The movement of the diaphragm activates the pushrod that further operates the mechanical parts of the brake.

Types of air brakes in a truck

Although there is a single brake assembly in trucks to stop it, it has three power sources acting on this brake.

The three possible scenarios can be air pressure in service brakes, manually operated large spring in parking brakes, and automatically released compressed spring in emergency brakes.

Parking brakes

There is an additional compressed spring with an air brake assembly that drivers use to park a vehicle.

It operates by a push-pull button in front of the dashboard. Functioning the button releases the air pressure from springs, and large, powerful springs force the parking brakes to lock the wheels.

Therefore, there is another name for spring brakes for parking brakes due to their working mechanism.

Service brakes

It is the regular brakes that drivers use to slow down or stop the vehicle during driving. It works by using the air pressure compressed in the reservoir tank.

Emergency brakes

During driving, if service brakes fail due to low air pressure, the compressed springs of parking brakes automatically release due to loss of air pressure and apply the brakes.

In this way, parking brakes act as emergency brakes to safely stop your vehicle, avoiding an accident.

How do air brakes operate?

Here is a quick summary of the S-cam drum air brake’s working mechanism from driver’s action to vehicle stops.

● When you press the brake pedal, the triple valve actuates and allows airflow from the storage tank to the service lines.

● Compressed air enters the brake chambers through the valve and actuates the diaphragm.

● The diaphragm connected with the pushrod moves it, which drives the slack adjuster and causes the S-cam to rotate.

● S-cam pushes the brake shoes or pads against the rotating drum surface.

● Friction between the drum and pads causes the wheels to stop, thus stopping the vehicle. 

Comparison of hydraulic brakes with Electric Brakes

Electric, hydraulic and pneumatic brakes are different from each other based on transmission media, their applications, and financial impacts. The chart given below compares a few general aspects of all three types.

Properties Air Brakes Hydraulic Brakes Electric Brakes
Working Principle Compressed air activates the brake shoes against drums A hydraulic fluid exerts hydraulic pressure on the braking components. Electromagnet actuates the braking parts to stop the vehicle
Transmission Media Compressed-air via rubber pipes Incompressible hydraulic fluid via reinforced flexible pipes Electric current via wires
PowerSource The vehicle engine runs the compressor to suck air from the environment. No power source 12V battery of the vehicle
Applicable Vehicle Weights <15000lbs 15000lbs-30000lbs >30000lbs
Application Light vehicles, campers Smaller trucks, vans Heavy trucks, commercial bus,

Railway trains

Installation Cost Low Medium High
Operational and Maintenance Cost Low High Low
Reliability of Operation Medium Low High
Sensitivity High High Low

 Why do you need an endorsement or license to operate air brakes?

Their operation needs a thorough understanding of the pneumatic system and its working mechanism.

The impact of applying the brakes is loud, and with a slight push of the brake pedal, you feel a substantial braking force on your vehicle compared to hydraulic brakes that have a smooth operation.

Therefore, drivers need complete training and earn a license or certificate to ensure the vehicle’s smooth stopping. 

Air brakes courses train the drivers to pass their exam for a commercial driver license (CDL) or license to operate the bus or truck with these brakes installed.

Various US states have regulations to have CDL for driving vehicles above 12 tonnes weight rating. 

General preventive maintenance instructions for pneumatic brakes

These are reliable due to the availability of redundant measures available in failure or air leakage.

However, their smooth and efficient operation depends upon the preventive maintenance and regular inspection of the various parts.

● Regularly check the compressor for any loose bolts or belt connection with the engine.

● Perform the manual adjustment of brakes with a slack adjuster every month to compensate for any wear and tear in brake parts.

● Periodically inspect and replace the worn-out parts like drum and shoes.

● Inspect daily the air pipes for any kinks or wear and tear to avoid leakage.

● Before driving, check the air pressure to be within acceptable limits.

● Check the operation of emergency brakes by releasing the air pressure.

● Drain the storage tank regularly for moisture and oil as its excessive levels can damage the other parts.

● Check the pressure buildup rate of the compressor and its drop rate.

● Calibrate the pressure governor for the safe operation of the compressor.

● Calibrate the safety valve every quarter to ensure the safety of other parts.

● Clean the pneumatic valves every quarter for proper coordination among them.

Can Air Brakes Operate with Air leakage in the system?

Air brakes’ design is fail-safe, and they can operate despite any minor leaks in the system.

In case of air leakage or low air pressure up to acceptable limits, the compressor’s continuous operation will keep air supply to the service lines.

This air pressure will be sufficient to apply the brakes at low speeds.

In case of significant leaks or drastic air pressure reduction, the spring brakes will automatically actuate and stop the vehicle until you take any remedial measures to find the air supply system’s malfunctioning.

Transmission Delay in Air Brakes

These can suffer a transmission delay of 500-700 milliseconds between the driver pressing the braking’s pedal and execution.

It is commonly known as brake lag. It is the time that air takes from the reservoir tank to the brake chambers.

This phenomena you will observe only in pneumatic brakes as lines in hydraulic brakes always remain filled with the hydraulic fluid.

While in the case of electric brakes, the current reaches the magnet in no time. 

This brake lag enforces the truck drivers to keep an increased safe distance from the front vehicle.

Reasons for Locking of Air Brakes

The common causes responsible for locking air brakes are poor adjustment of brake shoes, low air pressures due to leakage in pipes, high moisture and dust level in the air causing choking of valves, and improper adjustment valves.

Sometimes malfunctioning of various valves causes an imbalance in air supply to the different valves.

It causes brakes on some wheels to operate before the others, resulting in high stresses on truck axles and damages.

With proper maintenance, you can resolve these issues in the truck to ensure the efficient operation of air brakes.  

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