Don't Let Your Brakes Fail You

Brake system safety violations are the No. 1 reason heavy trucks are declared Out of Service by the DOT. Out of adjustment brakes usually top the list of problems that result in vehicles being removed from service, notes Robert Braswell, technical director, Technology & Maintenance Council (TMC), American Trucking Associations. Proper adjustment goes a long way toward alleviating problems.

Incorrectly maintained or adjusted brakes increase stopping distances and can cause costly and potentially dangerous component failures. Air brake systems are made up of many separate parts, all of which must be in good condition to provide reliable operation.

Inspections catch problems early. "In addition to visual driver inspections, technician inspections are important and should be performed on a regular basis," says Joe Kay, director, brake engineering, Meritor. "During these more thorough inspections, technicians should check linings for wear rates, cracks and fractures. If a problem is detected during an inspection, the vehicle should be taken out of service immediately. Brakes should be examined to determine the cause of the problem and corrective action should be taken."

A track record of reliability

Air brake systems are used on heavy trucks because they can reliably stop an 80,000-lb. load in a short distance. And unlike hydraulic brake systems – where a leak causes a loss of braking performance – a leak in an air brake system actually causes the brakes to be applied. Air pressure is required to release the brakes.

The main components of an air brake system include:

* a compressor that builds up and maintains the air pressure,

* a reservoir that stores the compressed air,

* a foot valve that transfers the compressed air from the reservoir when it is needed for braking,

* brake chambers that transfer the forced air to the mechanical linkages,

* and brake shoes and drums that create the friction needed to stop the vehicle.

The systems also utilize service and parking brakes. "Vehicles with air brake systems are equipped with both spring and service brake air chambers," says Kay. "The service brake chamber provides the input force for normal stopping power of the vehicle based on driver input to the brake pedal. The spring brake is operated by a large spring and used when the vehicle is in park or in emergency stopping situations." The yellow diamond-shaped button on the dashboard releases air from the system, actuating the spring and applying the brakes.

Air brake systems have built-in redundancy. "The integrity of a dual air system is insured by check valves," says Tom Soupal, principal engineer, valves, Meritor WABCO. "A check valve failure in the primary or secondary circuit can compromise the system. To test check valves, open the drain valve on the wet tank. Pressure in the primary and secondary tanks must not leak."

Environmental factors

Brake system component life is influenced by the environment. An off-road environment will shorten service intervals, according to Thomas Macmenemy, engineering manager, Daimler Trucks North America. "Inspections become more frequent in order to catch accelerated wear before an out of service condition occurs," he states.

Off-road conditions present unique challenges. "Cleanliness of the system will affect wear rates. Dirt and debris between the lining and braking surface will result in shorter lining and drum/rotor life," says Kay. "Rubber boots and seals can also be damaged by debris."

Vibration and harsh impacts can lead to unique fractures and leaks. "It is important to evaluate a vehicle's operating conditions and plan inspections and maintenance accordingly," says Kay.

Also use components that are up to the task. There are normal and heavy-duty versions of most components. TMC recommends that fleets use the highest quality component available for the application.

Adjustment is the Achilles' Heel

Proper operation of the push rods and slack adjusters is mandatory to maintain the position of the brake shoes relative to the drum. Check the push rod actuation out of the brake chambers. Springs in these chambers can break or become weak over time. If the spring breaks, the rod will not retract when the brakes are released and the brake linings may drag against the drum.

Air brake adjustment should be verified as a regular part of any pre-trip vehicle inspection. Check the slack adjusters. "Look for broken or missing parts," says Macmenemy. "Check chamber stroke. If the orange band on the push rod is showing, have the brake serviced."

On drum brakes, the slack adjuster is external and should be checked to ensure the air chamber stroke is within the acceptable operating range. "If range of motion is greater than the specification, or the orange marking on the air chamber rod is showing, the slack adjuster should be inspected to make sure it is functioning properly," notes Kay.

However, he cautions, "A technician should never manually adjust a slack in order to bring it back into adjustment. A malfunctioning slack adjuster should always be inspected by a certified mechanic to find the root cause of the problem, and suspected parts should be replaced. If a failure is detected, the slack adjuster should always be replaced with the same size, brand and style."

There should not be any play or looseness in the slack adjuster. In addition, TMC does not recommend sealed-for-life brake adjusters for severe vocational trucks, since contaminants are likely to work their way into the mechanisms of the adjuster and increase the wear of those components.

Find the right match

Inspect the brake lining during the pre-trip inspection. "Look for cracked, broken lining blocks, uneven wear left to right on an axle or between forward and rear axles on a tandem," says Macmenemy. Make sure there is no scoring or that the drums are not out of round.

Brake linings must be carefully matched. Any difference can result in uneven brake wear and reduced braking performance. Broken or weakened return springs will also prevent the brake shoes from retracting away from the drum.

According to TMC both wheel ends of each axle (driven or non-driven) should be equipped with identical drums and linings. All four ends of a tandem axle should also be equipped with identical drums and linings. Higher temperature-resistant linings, shoes and assemblies are needed for severe vocational applications.

Inspect for leaks

Periodically check for air leaks. "The best method to determine if you have an air leak is to occasionally park the vehicle and monitor the pressure gauges," says Macmenemy. "Loose fittings are going to be responsible for the majority of air leaks."

Any leaks should be noted during the required pre-trip walk-around. "The operator should listen carefully for audible leaks," says Soupal. "In addition, the vehicle can be tested with the service brakes applied. Apply the foot valve and let the pressure stabilize. If the primary or secondary reservoirs leak more than 1 psi per minutes, the leak should be located and repaired."

Soupal adds, "Valves may develop leaks due to contamination and other causes, but the most common sources are the fitting and line. Inspect the system for kinks and potential abrasion spots. Be especially careful with lines that might come into contact with any surface during full stroke of the suspension system."

For most trucks, as the brakes are used, the air compressor should kick in at approximately 100 psi and kick out at approximately 125 psi. If the pressure drops to 60 psi, a low air warning light and/or buzzer should be triggered. As the pressure drops lower, the spring brakes apply between 20 to 45 psi.

"All trucks must be equipped with an audible low pressure warning," says Soupal. "To test, use the primary air reservoir drain valve to drain the pressure and insure that the warning is activated at 60 psi or above. Repeat with the secondary tank."

Don't forget the parking brake. "Spring brake chambers provide a mechanical means to park the vehicle," says Macmenemy. "Check them daily for leaks in the service and park side of each chamber, and look for broken or damaged components due to impact from road debris."

Keep moisture out

Moisture and air brake systems don't mix. "Water in the pneumatic system can freeze in cold weather, and excess moisture can degrade brake actuation performance and shorten service life of air system components," says Braswell.

"Ignoring moisture in an air system is detrimental to the operation of the vehicle," agrees Gen Ekola, senior project engineer, Meritor WABCO. "The moisture is not just water; there are traces of oil and contaminants from the compressor mixed in. As soon as the mixture of moisture, oil and contaminants starts to fill the tanks, the mixture will travel through the air brake system collecting at any low spot in the various valves in the system. If there are any steel components in the air system – governors are an example – corrosion will set in, preventing the part from functioning."

Freezing of this mixture can damage the valves, as well as prevent the supply of air to the brake system. "After collecting in a valve, the water and oil mixture will eventually degrade the valve operation," Ekola notes, "changing the braking characteristics of the vehicle and potentially creating high costs in part replacements."

All air brake systems should be equipped with a dryer cartridge. "Dryers remove both water and oil from the air system," explains Macmenemy. "This results in cleaner air which will reduce component wear and the possibility of ice forming in the air lines and valves."

Even with this safeguard, he advises draining the air tanks daily. In fact, some states actually require it.

"Many states have DOT requirements that indicate air tanks are to be drained daily," says Ekola. "This is especially true if the driving location is in a warm and humid locale, such as southern states with water borders. In a state that does not have a requirement, the tanks should be drained weekly to verify the air dryer is operating."

If water is found, corrective action must be taken. "First, make sure the tanks are drained completely to remove all the moisture," says Ekola. "Then check the charging/drying system to make sure the system is operating correctly. Check for leaks in the air system and check the governor to ensure a signal is being sent to purge the air dryer.

"At the same time, turn off the compressor from pumping," Ekola continues. "Verify the compressor is able to fill the reservoirs in a timely fashion (check FMVSS 121 for required times) and that the air dryer can purge/regenerate for at least 30 seconds or more between pump cycles."

Also verify that the drying cartridge is functioning properly. "If you happen to notice water in the air tanks, the first place to look would be the air dryer to verify that the desiccant cartridge isn't saturated," says Macmenemy.

"The most important step to take is to replace the air dryer desiccant cartridge, especially if no problems with the air system are found," Ekola emphasizes. "Many times, when a desiccant cartridge becomes saturated, the desiccant may no longer be able to regenerate and the only solution is to replace the cartridge."

Choose repair parts wisely

Consider differences in the friction materials when it is time for replacement. "OEM brake lining is designed to balance the work load between steer, drive and trailer brakes," Macmenemy points out. "OEM lining meets or exceeds FMVSS 121 requirements."

FMVVSS 121 relates to minimum federal performance standards for the entire braking system and applies to new vehicles only. Aftermarket friction materials are not regulated by FMVSS 121. Make sure the performance meets or exceeds the OEM brake linings.

According to TMC, the torque values of an aftermarket lining should approximately match that of the original equipment it is replacing. The Performance Review Institute (PRI), an affiliate of SAE International, formed a Brake Lining Review Committee to review results of FMVSS 121 dynamometer performance tests conducted by qualified laboratories. Results are available at