Slack adjusters-or brake adjusters as some call them-are devices used to maintain the proper clearance of the brake shoe friction material to the brake drum.
According to Randy Petresh, vice president-technical services, Haldex, “The primary purpose of a brake adjuster is to adjust the brakes automatically while they are in service. The brake adjuster maintains the proper running clearance between the brake shoes and the drum which, in turn, will minimize changes in brake chamber stroke.”
He added, “As the brakes wear, stroke constantly changes and increases unless the brake adjuster is there to compensate for the wear.”
While brake adjusters do not require a great deal of maintenance, technicians often will adjust them. John Hawker, engineer, Bendix Spicer Foundation Brake, said, “Do not adjust them. And I want that in bold and caps.” He explained that it is a dangerous practice to adjust automatic slack adjusters because an out-of-adjustment slack adjuster could mask an underlying brake system problem, including worn components within the wheel end or a broken return spring.
So what is the proper way to care for a slack adjuster and what should you do when you find an adjuster that is out of adjustment?
“Repositioning the adjuster and sending the vehicle out is not fixing the problem,” Hawker said. All you are doing is addressing the symptoms, not dealing with the cause.
Even before addressing maintenance concerns, make sure the brake adjusters are installed properly. “The installation is critical,” Petresh said. Therefore, whenever you do any wheel end or brake maintenance that disturbs the adjuster, make sure to reset it according to the way it was when it was installed originally on the vehicle.
The first thing you should do when a vehicle is in your shop is perform a visual inspection. “This is important because of where the adjusters are located: Underneath the axle, close to the road. They take a beating when it comes to road debris, contamination, corrosion, salt and mud,” Petresh said. “Doing visual inspections helps to make sure there isn’t any damage to the seals, the boots, the operating mechanism or the attaching points.”
Hawker advised making sure you properly lubricate the adjusters during the vehicle’s normal chassis lubrication. However, before lubricating the slack adjuster, you should record the home position-with the brake released, it is where the clevis is in relationship to the face of the chamber.
Next, using a pry bar, stroke the brakes until the lining touches the drum. This establishes free travel, which should be between 3/8 inch and 5/8 inch.
The next step is to build air pressure to 100 psi, shut off the engine, do a full brake application and then measure power stroke.
Power stroke should be no more than what the current Commercial Vehicle Safety Alliance guidelines specify for the type of chamber used.
If any of the measurements are out of spec, disassemble the wheel end to determine the real problem.
If all the measurements check out, then lube the S-cam and the slack adjuster according to the manufacturer’s established guidelines.
These procedures are simple enough, but some technicians still persistently try to adjust the automatic brake adjusters.
“People try to take the quick and easy way out and adjust the adjusters. They tighten them so the stroke decreases and then send the truck back out,” Petresh said.
“If they do that they are going to end up with the same problem within a few days.”
He added, “If a brake adjuster is not maintaining the required adjusted stroke it is because something else in the system may be a problem.
Petresh, who reviews warranty returns for Haldex, said that 40 percent of the returned adjusters are fine and have nothing wrong with them.
“When the technician sees that the brake adjusters are out of adjustment, he needs to determine the real problem. Does he have a cracked shoe? A flat spotted roller? A damaged cam shaft? A broken chamber bracket? An out-of-round drum?
“All of these foundation brake wheel-end issues have an impact on chamber stroke and brake adjuster characteristics,” he said.
There are a few things that can go wrong with the actual adjuster itself. For example, it can get damaged or hit by something like road debris, curbs, etc., or it can become rusted and corroded. Proper lubrication will help with preventing rust and corrosion.
According to Hawker, proper lubrication for all slacks includes 100 percent purging of the system. “Each manufacturer has its own procedures, but all contaminants must be purged and this includes water, chemicals, dirt and air,” he said. “You want the body of the slack adjuster to be 100 percent filled with an NLGI-Grade 2 lube. Do not use molybdenum disulfide or a light-duty grease.”
According to Hawker, another important item to check is the clevis pin to see if it moves. “If it does not move, do not send the truck out. Instead pull the pins out and clean them with a wire brush on a grinder, then apply anti-seize compound,” he said.
If you do not apply anti-seize compound, rust will develop and you will end up with a seizure of the pin to the slack adjuster or a seizure of the pin to the clevis.
“At that point, the slack adjuster stops adjusting because the pivot point no longer is free to move.”
He continued, “If you seize up a pivot point, you are going to reduce the ability of the slack adjuster to maintain adjustments. And you will put a bending moment into the push rod arm of the clevis.”
As you can see, if slack adjusters are not maintained properly, they will lead to more serious problems down the road and even can result in a truck being taken out of service during a roadside inspection.
But it is also important to keep in mind that a slack adjuster that is out of adjustment usually is the result of a problem elsewhere in the vehicle’s braking system.
When a vehicle comes into your shop with a slack that is out of adjustment, take the time to find the real problem and never manually adjust an automatic slack adjuster. That will not fix the problem and in fact can mask a more serious braking system issue.
