Maintenance and repair of brakes has always been a vital, and sometimes overlooked, part of a heavy-duty PM program.
Today, it's more important than ever, as new de-icers cause rust-jacking of brake linings and corrosion-induced failure of other brake parts, and as we look ahead to new stopping-distance requirements expected to be announced at any time for on-highway air-braked tractors.
Making things more challenging, automatic slack adjusters (also called automatic brake adjusters) are often misunderstood by technicians.
Brake maintenance, particularly maintenance intervals for heavy-duty trucks, depends more than anything else on the vehicle's duty cycle. There is a world of difference between a refuse truck that experiences 200 to 300 heavy brake applications in a shift and an over-the-road truck that may have two applications daily - one for lunch and the other to shut down at the end of the day.
Oddly enough, the easy-duty application may dish up more problems than severe service, because very little heat is generated in the brakes. Over-the-road trucks might see 125-150 degrees in the brakes on average, where a refuse truck may well have brakes running 450-500 degrees. These severe applications result in shoe lining wear-out in as little as two months, with drum replacements maybe every three times the shoes are exchanged. Similarly, the other brake parts - chambers, pushrods, camshafts, springs and particularly automatic brake adjusters - get a good work-out and get inspected frequently so there's less chance of corrosion and seizure. And service intervals are predictable, so service can be scheduled.
On over-the-road trucks, however, brake service is usually done when mechanics routinely lubricating the chassis peer through the access holes in the dust shields - when they are fitted - to discover the linings are worn thin. For a conventional S-cam brake this might be around the 250,000-300,000 mile mark. (Good drivers, though, can make a set of linings go twice as far.)
But this is a somewhat hit-and-miss inspection process, says John Hawker, the primary field engineer for Bendix Spicer Foundation Brake. Far better, he says, to use a wear gauge provided by the brake manufacturer. The correct procedure is to note the date the truck is placed in service and the mileage at which 50 percent of the brake block has been worn away as indicated by the gauge. So when a 7/8-inch block shows 7/16-inch remaining when measured with the gauge, the mileage to full lining wear-out can be estimated fairly accurately.
Hawker uses the example of a 50 percent indication at 300,000 miles, at which point he strongly suggests pulling drums at 500,000 to 550,000 miles to see what is going on inside the drum, how much lining is left on the shoes and the condition of the brake parts.
Automatic Brake Adjusters
The most immediate indicator of a problem is a brake stroke out of range. Automatic slack adjusters, or ASAs, also called automatic brake adjusters, have been required for trucks and tractors since 1994 and trailers since 1995. For tractors, it is likely that between 93 percent and 98 percent will have auto slacks. For trailers, this percentage could be considerably less, since the regulation came in later and trailers stay in fleets a lot longer.
Trailers that have seen brake work may have been retrofitted with the devices, since using them results in far fewer DOT out-of-service events -a real saving to a fleet. A Canadian study showed vehicles without auto slacks are twice as likely to be put out of service for brake stroke outside of mandated limits.
There are two things really to be understood about automatic brake adjusters. One: They work, and work well - until they stop working. Two: When they stop working they must never be manually adjusted, because the ASA is telling you there's likely something else wrong with the brake.
Several crash fatalities have occurred because of manual adjustment of automatic brake adjusters, and the National Transportation Safety Board has come out with a very strongly worded prohibition against tampering with the auto slacks.
Drivers have a duty to inspect brakes for proper adjustment during a pre-trip inspection. Drivers are qualified to inspect brakes - it is in Section 5 of the CDL Study Guide, accepted by all 50 states. However, a driver is actually forbidden from working on truck brakes unless he or she has the training or experience required under FMCSR Part 396.25.
So if a driver finds a brake out of adjustment, he has a duty to report it and get it fixed by a qualified mechanic.
How the driver does this inspection is a matter of some debate. Daniel Judson, technical director of Brake Sentry, says hauling on the slack adjuster isn't the right way to do it, even though that is the prescribed procedure in the regulation. His solution is to put a stroke indicator like his company's product on the brake chambers. Then, a driver can see during the walkaround if the brake pushrods are stroking too far out of the chambers, getting to the dangerous full stroke where they can no longer apply the brakes.
Overstroke: Symptom, not Cause
Brake chamber overstroke is the first and most obvious sign that something is wrong at the wheel end. Automatic brake adjusters out of their stroke range are most often a symptom, not the cause, of a problem, says ArvinMeritor's Joe Kay, engineering manager for Brake Systems. "The only time an ASA should need adjustment is if some kind of brake work has been performed and the adjuster has been backed off manually," he says. Then the correct procedure must be followed.
Bendix's Hawker says that an adjuster with a clutch mechanism, such as a Haldex, Gunite or Bendix, must be backed off only as far as is necessary to remove the drum. In fact, he says, if the brake drum is being removed, the automatic brake adjuster should also be removed, cleaned, checked for operation and lubricated before reassembly onto the brake assembly.
In terms of general maintenance, during a brake inspection, the mechanic should place the appropriate wrench on the adjuster nut while a buddy applies the brakes. The wrench should index as the brake applies. If it fails to move, there is something wrong with the adjuster and it must be removed for service. If the wrench swings, the adjuster is working and should be left alone.
Some fleets, Hawker says, have mechanics put a wrench on the adjuster and manually adjust them up whenever a truck comes in for service. This is absolutely wrong, as the technician is overloading and slipping the clutch, which will ultimately damage the automatic adjusting mechanism.
Don't Skimp on Lube
Problems with S-cam rollers or S-cam bushings might cause issues with automatic brake adjusters overstroking. S-cam bushings are susceptible to corrosion if seals are defective or if an inferior grease is used in maintenance. Kay points out that the ArvinMeritor long-life, low-maintenance wheel ends use high-quality synthetic grease. He recommends these are the types of lubricants to be used on camshafts.
Hawker says to use the best lubricants available on automatic slack adjusters and to keep pumping grease until all old grease is purged from the component. The cheap dollar-per-pound grease will wash out quickly, where a $6 per pound synthetic will stay in place and do its job. This is not a place to skimp on quality, both Kay and Hawker say.
Since the clevis points are critical in the process of detecting stroke or clearance, the clevis pins must always be free in any automatic brake adjuster. For that reason, they should be treated to an anti-binding lubricant whenever the truck is in for a chassis lube service.