Truck Topics

Brake Basics

The Commercial Vehicle Safety Alliance, as part of its recent Brake Safety Week in late August, pointed out the following facts:

• Results from a number of National Transportation Safety Board investigations into high-profile commercial vehicle crashes have revealed problems associated with brake operation, inspection and/or maintenance.

• Results from the recent Large Truck Crash Causation Study, sponsored by the United States Department of Transportation, indicated that brake problems were present at the time of the crash in 29.4 percent of the cases.

• Brakes comprised the largest percentage, 56.6 percent, of vehicle Out-of-Service Violations cited in roadside inspections conducted during Roadcheck 2007. 

• Nearly 9 percent of all brakes equipped with manual slack adjusters and  four percent of all brakes equipped with automatic slack adjusters  are placed out of service.

Here are basic things you can do to make sure your brakes are working correctly.

1. Brake Adjustment

Automatic slack adjusters were supposed to solve brake adjustment problems. They are designed to regularly advance the stroke of pushrods to compensate for brake lining wear. But roadside inspections continually find autoslacks not operating because they have been damaged internally or are no longer in a physical position to work.

One of the big reasons for this is that drivers and mechanics try to adjust the slack adjusters – and they shouldn't. Overadjusting damages the ratcheting mechanisms and makes them useless.

Autoslacks do need regular maintenance, including checking of mounts and lubrication, but don't try to adjust them. If an autoslack seems to have quit working, chances are it's a problem in the brakes themselves. Automatic slack adjusters are designed to maintain a specific shoe-to-drum clearance. If the foundation brake components wear, the slack adjuster can’t maintain the proper adjustment.

Like any other part, ASAs can malfunction or wear out. If your brake issue truly is the autoslack, you're better off removing it and replacing it with a new one, rather than trying to adjust it.

2. Regular Inspections

Just because you have automatic slack adjusters does not mean you never need to look at your brakes. Perform a visual inspection of slack adjuster adjustment as part of regular pre-trip inspection activities. At the same time, make sure all brake chamber push rods stroke out and release without hesitation.

Properly checking for brake adjustment is really a two-person job. Each of the actuator brakes needs to be checked for proper stroke when the brakes are applied to full pressure -- about a 90-psi brake application. This is more effective than just grabbing the slack adjuster arm and trying to move it.

During your inspection, check for worn brake shoes and cracked brake drums. Look for signs of brake lining wear, such as lining thickness and any signs of cracks that can be seen from the edge or the side of the brake shoe. You can expect some cracking as the lining gets near the end of its life. However, if there’s still plenty of lining left, but you’re seeing cracks or gaps between the block and the shoe, it could be a sign of “rust jacking” -- corrosion often caused by new anti-icing chemicals.

3. The Right Brake Linings

When brake linings need replacing, don't just buy the cheapest thing that will fit. The companies that supply brake lining to truck manufacturers spend a lot of time and research designing friction materials specifically for  certain chassis and certain conditions, including weight ratings and temperatures.

Replace with the wrong type of brake linings, and the relined brakes can have reduced braking output, causing brake imbalance -- a shift of work to brakes on other axles. This not only reduces the overall stopping capability of the vehicle, but also can lead to shorter lining life, and overheated and even cracked drums as those brakes are overworked.

Choosing the right brake lining isn't always easy. Performance of new equipment is regulated by the National Highway Traffic Safety Administration. But the Federal Motor Carrier Safety Administration, which regulates in-service equipment, has no regulations in place that require replacement brake materials to meet the same standards. As a result, there's an almost bewildering array of options, including cheap will-fit parts from offshore that may not perform as advertised.

The most foolproof way to make sure your brakes performed as designed is to replace worn brake linings not just with new linings, but with new brake shoes and linings from the original equipment manufacturer. Even some major fleets have stopped using relined brakes. While this costs more per replacement than a typical reline brake job, these fleets have found that in the long run, this strategy leads to the lowest cost per mile.

While this approach is gaining more fans, most truck owners are still going to want to replace just the linings. To get the best performance, the first step is to find out from your dealer what the truck was built with initially.

To help choose the right lining, you can refer to the Technology and Maintenance Council's Recommended Practice 628. This provides a database of materials that have been tested and approved for aftermarket use. You can download the most recent listings from the Performance Review Institute, which administrates the testing program, at www.pri-network.org/Brake-Lining-Program.id.29.htm.

Most importantly, don't buy just on price. Saving a few cents on the lining can cost you a lot more if a DOT inspection finds a problem with it and you have to do unscheduled maintenance. A good strategy is to stick with recognized name brands from companies with a good reputation, and work with a company that has the knowledge to help you select the right replacement lining.

Deborah Lockridge is senior editor of Heavy Duty Trucking and RoadStar magazines and a member of the Technology and Maintenance Council, with 17 years of experience covering the trucking industry.