In my early years as a young mechanic, my wife, Linda, would come to the shop where I worked. She came down to give me time with her and our children. The kids would play in the tire rack, climbing through the tunnel of tires while Linda helped me do whatever I needed help doing. One of the many projects we worked on late into the evenings were brake jobs. In the ’80s, all the brakes I saw were drum brakes. Every time a brake job was needed the entire wheel end had to come off. This meant at least one hour for each wheel.
Times have changed. With the advancements in disc brake technology, air disc brakes are now the coming norm. As with almost everything in the trucking industry, disc brakes were very slow to gain acceptance and commonality. Now brake friction materials (brake pads) can be changed rather quickly.
Now we have disc brakes. My life has become much easier when the brake service is needed. The wheel assembly must be removed, allowing access to the brake caliper that carries the brake pads. The main parts of a disc brake operating assembly include the caliper, rotor and the pads. The rotor is attached to the hub and rotates with the wheel. The caliper is attached to the spindle and does not move with the wheel. The caliper “floats” on a support attached to the spindle, carries the brake pads and keeps the proper clearance allowing free rotation of the rotor between the pads. As air is applied thru the brake pedal, the caliper squeezes the rotor between the pads creating friction, this slows the rotation of the rotor, thus the wheel and in turn the vehicle.
Today we changed the pads on the steer axle of our 2015 Freightliner Cascadia. At over 500,000 miles, we have sold the truck and wanted the new owners to be worry free on maintenance for some time. The pads were not worn to the point of “need to be changed” but were at about two thirds. There is a vertical groove cut into the center of the pad that is a wear indicator. Disc pads friction material starts at 21mm, pads must be replaced a minimum of 2mm friction material. A pad kit comes with all necessary hardware needed to do the job on an axle, two-wheel ends.
Safety first, think about what you are doing and do what is needed to keep you safe. That said, jack up the wheel you are servicing and place jack stands under the axle. Remove the wheel/tire assembly. Now you can see the caliper. Some calipers have shields that need to be removed, our doesn’t. On the back side, the brake chamber side, there is a rubber dome-shaped cap, remove this, it will be replaced. Under the cap is a 10mm hex that is used to “backed off” the brakes. This hex is made of a softer material that is designed to shear off if too much force is applied while backing off the brakes, this is to prevent damage to the auto adjust the portion of the caliper. Turn the hex gently in one direction, if there is a sharp “click”, you are turning the correct direction. If the pads get tighter on the rotor, STOP turning or the hex will shear. Not to worry, there are new shears in the kit. Keep clicking, you will see the pads are getting looser within the caliper. Watching the pistons that push the pads to retract, slow down as the pistons near fully retracted until they are retracted.
Now the pads can be removed easily, remove the spring clip from the pad retainer pin, then the washer. Depress the pad retainer, by hand or using a pry bar and remove the pin then the pad retainer. The pads will lift out now. The rotor can be inspected for wear and/or heat cracks. The caliper can be checked for ease of movement, it should float with minimal effort, if not repair as needed. The new pads need to have the holder springs installed, then placed in the caliper, friction material toward the rotor. Then reverse the disassembly process, pad retainer, retainer pin, washer, spring clip.
Adjust the caliper with the shear hex gently until the pads tighten on the rotor, you should be able to turn the rotor but it will be difficult, then back off the brakes two clicks. The brakes are now properly adjusted. Put the wheel back on and do the rest of the wheel ends the same way.
This wheel took about 15 minutes to replace the pads including removing and installing the wheel, life is good. Disc brakes are lighter and, in my opinion, provide better braking, they also cost less than drum brakes to replace. Less maintenance, nothing to grease, no slack adjusters to fail or measure stroke on.
That's all I have, for now, be safe out there.