Heavy Truck Air Brake System Failures Expert Overview

Brake systems on commercial trucks function differently than conventional passenger car brakes and have different requirements for inspection and maintenance. Here we review the basics of heavy truck air brake systems for attorneys who may be involved in litigation involving commercial trucks. 

Commercial / Heavy Truck Air Brake Systems

Unlike traditional light duty truck or car brakes which use hydraulic fluid to operate, brake systems on commercial trucks use air pressure.

The air pressure is supplied by an onboard compressor driven by the engine to create air pressure between 100 and 120 psi. The air pressure is used for both the parking brake and the service brakes (i.e. normal brakes or foot brakes).

Loss of air pressure within the braking system can result in brake failure, overheating in the braking systems which can result in fires, and loss of vehicle control. 

Brake failures in heavy trucks are different than passenger cars in that when air brakes fail, they fail on, i.e. brakes applied. This is because of the fail-safe design of Spring Brake Chambers in an air brake system.

Within the brake chamber there is a heavy-duty spring that is constantly pushing to apply the brakes (slow the vehicle). To overcome the spring brake in the brake chamber, air pressure is required to allow the wheels to roll. The air pressure within the system works to counteract the spring, allowing the vehicle to roll freely.

When the service or parking brakes are applied, air pressure is released from the system, which allows the spring to extend the pushrod, applying pressure to the shoes which interact with the friction surface of the drum, slowing the rotation of the wheels. In the event of sudden loss of air supply to an airbrake system, the spring brake is applied almost instantly, akin to slamming on the brakes. If the speed is high enough the wheels will lock and the tires can skid uncontrollably.

However, even partial loss of pressure can lead to negative outcomes.

Truck Loss of Control Incidents

When a driver presses the brake pedal on a truck they are allowing air pressure to be sent to the service brake chambers at each wheel which is what is applying the brakes.

A leak in an airline can lead to uneven braking as air pressure will not reach all the brake chambers consistently. An out of balance brake system can cause a left side brake to apply more force than a right side brake, potentially causing a commercial truck to be put into a “jackknife.”

A five axle commercial truck and trailer can gross 80,000 pounds, and overweight permits can be obtained to allow operation of over 100,000 pounds.

Having a brake system that is not braking evenly can make slowing down and stopping extremely difficult and can cause the operator to lose control of the vehicle. The trailers that these trucks pull are equipped with the same type of brake system, operating in the same manner discussed above.

With all the mechanical and moving components that go into a commercial truck brake system it is vital that these components are maintained and function properly.

Heavy Truck Brake Fires

If there is any leak in the tubing that supplies air to a wheel’s spring brake diaphragm, that brake can start to be applied without input from the driver. If this brake shoes continue to rub against the brake drum for an extended period, it can generate enough heat where the axle grease can start to boil and may cause a fire.

Inspection Requirements for Heavy Truck Braking Systems

Heavy truck braking systems require routine maintenance and inspection to operate safely. This includes regular greasing to all the grease fittings on these components, inspecting the pivot joints for proper movement, and inspecting the slack adjusters to ensure the brakes are staying in adjustment. This is crucial for equal braking to all wheels during a braking event.

A trailer that is in tow often weighs more than twice the weight of the truck. Neither the tractor brakes or the trailer brakes alone are designed to stop a tractor trailer combination.

If the trailer brakes are not functioning properly, this can cause the trailer to push the truck making the rear end swing out, causing a “jackknife” condition.

During a forensic investigation, great detail goes into inspecting the brake system of a heavy truck, including:

• Visual inspection of all airlines and components
• Inspection of the compressor and air dryer system that helps create consistent and dry air for the brake system.
• Measuring brake shoe material, looking for cracks or hot spots in the drums themselves
• Applying air to the brake system and having the brake pedal applied to allow measurements of all the brake chambers to determine the amount of travel each chamber has and comparing that to the amount of movement the chambers are designed to move as well as what DOT regulations allow.
• Additional components as needed for the details of the case

These inspections and procedures can be done even on a severely damaged unit that has been involved in a crash, providing evidence that may allow experts to determine the condition of the brake systems at the time of the incident.

Commercial Truck Driver & Motor Carrier Responsibilities

The Federal Motor Carriers Safety Administration (FMCSA) addresses inspection, repair, and maintenance of commercial motor vehicles in § 396 of the Federal Motor Carriers Safety Regulations (FMCSR).

§ 396. 1(a) Every motor carrier, its officers, drivers, agents, representatives, and employees directly concerned with the inspection or maintenance of commercial motor vehicles must be knowledgeable of and comply with the rules of this part. 

Driver Responsibilities for Pre/Post-Trip Inspections

When a driver performs a pre/post trip inspection there are several components to the air-brake system that a driver is required to inspect. First there is the visual inspection of the components such as the air compressor and belt, slack adjusters, drums or discs, linings, and hoses. Then a driver needs to ensure the brake system is functioning properly.

1. After starting the tractor, the air gauge should read between 120-140 psi when the system is fully charged.
2. The next step is to check for an air leak. This is done by turning off the engine, releasing the parking brake and depressing the foot brake, then timing the air pressure drop for a minute. During that time, the psi should drop less than 3 psi in a tractor or 4 psi in a tractor-trailer.
3. The next step is to inspect the low-pressure warning signal. This is performed while the truck is off, with the electric power on, by stepping on and off the brake. The low air pressure warning should come on before the pressure reaches 60 psi. Continuing to step on and off the brake, the air supply control should “pop out” between 20 to 45 psi.
4. To test the emergency brake, a driver would activate the trailer brakes and gently pull, to check the emergency brakes are on.

Motor Carrier Responsibilities

Motor carriers are responsible for ensuring that all vehicles in their fleet are systematically inspected, repaired, and maintained.

The FMCSA § 396.25 requires that individuals who inspect and/or repair brakes:

• Understand the service or inspection/repair procedures and can perform them
• Have knowledge of methods, procedures, tools and equipment
• Have had training and/or experience through an apprenticeship program or training and experience for at least 1 year.

Trucking Operations, Vehicle Engineering, & Crash Reconstruction Expertise

Our investigations of brake failure incidents typically involve a review of all available documentation from the driver’s pre/post trip inspections, incident reports, and maintenance logs for the truck and trailer.

Our trucking industry experts are frequently tasked with evaluating truck driver actions, driver logs, hiring practices, drug and alcohol testing programs, load securement, loading dock operations, lift truck operations & training, fleet management, and much more. Robson Forensic also employs vehicle engineering experts who understand the handling dynamics of heavy trucks and the physics required to accurately and dependably reconstruct collisions involving these massive vehicles.

For more information, submit an inquiry or call us at 800.813.6736.

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