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Stopping to the Beat of a Different Drum
Servicing Disc Brakes.
The effectiveness and simplicity of the disc brake compared to the drum brake has always been rather obvious. Initially, the cost of this design prevented its widespread use. However, as the popularity of disc brakes increased due to demand, the price decreased, making disc brakes affordable even on some economy models. Now, this design can be found on both the front and rear wheels on most of the vehicles built by FCA US LLC.
DISC BRAKE BASICS
Disc brakes work on the simple principle of pinching the brake rotor on each side in order to slow down and stop the road wheel. Hydraulic pressure pushes on a piston that, in turn, pushes the brake pad against the rotor. This force causes the caliper to slide and push the other pad against the other side of the rotor. On FCA US high-performance vehicles, such as the Dodge Challenger R/T, Charger and Durango, multiple piston calipers are used. These calipers apply hydraulic pressure to each side of the rotor.
Unlike drum brakes, heat is not the enemy of disc brakes. As the rotors become hot from use, the material expands. As a result, the pads don’t have to move as much to make contact with the rotors and brake fade is minimized. With drum brakes, the drum also expands as it gets hot, but this works against the braking effort. The brake pads are inside the drum and are pushed out to slow the road wheels. Therefore, the brake pads must move farther to reach the brake surface, increasing the possibility of brake fade.
Brakes, regardless of design, are wear items. At some point during the service life, the brake pads will have to be replaced. To a large degree, it depends on the driving style of the driver. If you drive fast and apply the brakes hard, the brake job comes sooner rather than later.
Brake pads should be inspected at least once a year. Remove the tire and estimate the amount of brake pad material that remains. Generally speaking, when the pad thickness (usable material) is reduced to about 0.040″ (1mm) at its thinnest point, it’s time for new brake pads. Always replace the inner and outer pad at the same time. Also, replace the brake pads for both front or rear wheels at the same time.
SERVICING DISC BRAKES
Replacing the brake pads is not the only task that should be performed. A thorough inspection is required to ensure that the new pads will stop the vehicle properly. After it has been determined that the pads must be replaced, raise the vehicle and secure it on jack stands or a lift and remove the road wheel. Then, remove the caliper and the pads. Before installing the new pads, the brake rotor must also be inspected.
Look for heat damage, excessive wear and any signs of scratching or scoring. Surface damage will require the rotor to be machined and re-faced. It’s also important to measure the thickness of the rotor. Using a micrometer, as shown in Figure 1, measure the rotor thickness in 12 different locations. These measurements should be within a few thousandths. If not, the rotor is warped. Depending on the severity of the warping, the rotor will have to be machined or replaced.
Cast into the hub of the rotor is the minimum operating thickness that must be held before replacement. It is very unlikely that a rotor would have to be replaced after the first or second brake job. But, then again, it depends on how the vehicle is driven.
Before the new brake pads can be installed in the caliper, the piston must be seated. This can usually be done with a large C-clamp or similar device. Once the piston is pushed back even with the surface of the caliper, the new pads can be installed. Once in place, the caliper can be re-installed. Don’t forget to replace the brake hardware and lubricate the caliper pins, as required.
REAR DISC BRAKE SERVICE
Rear disc brakes that are equipped with drum brake-type parking brakes are serviced in the same manner as the front brakes. Seating the piston can be accomplished using a C-clamp. On rear brakes that are equipped with a parking brake lever, collapsing the piston requires a different technique. Such a design is known as the Integral Parking Brake (IPB).
Now, a few words on the IPB. In this design, the caliper features a lever attached to a shaft on the outside of the caliper. The lever uses a ball-ramp mechanism to mechanically apply piston clamp force to the rear rotor when the parking brake is applied. As the lever and shaft rotate, the ball-ramp interface causes the shaft to lift and the piston to travel toward the rotor. This design includes an automatic adjuster mechanism that keeps the mechanical park brake mechanism in contact with the piston as the pads wear. The IPB is not used extensively in the FCA US LLC product line, but you will find it on the popular minivans built between 2008 and 2014.
Seating the piston on rear disc brakes equipped with IPB, a special seating method is required. The only acceptable method is rotating the piston back into the bore using special tool 8807-2. This is shown in Figure 2. The special tool is item (1) in the illustration. Insert the lugs on the tool into the notches on the face of the piston. Using another special tool 8807-1 (2) and a ratchet (3), thread the piston in a clockwise direction back into the caliper.
CHECK FOR CORROSIVE
In those parts of the country where winter is winter (snow, ice and salt), the IPB needs to be checked for corrosion.
Figure 3 illustrates the IPB design. Item (1) is the return spring, (2) is the lever and (3) is the anchor bracket. As you know, any part that moves must not corrode. In harsh environments, that is what can occur with this design. If the parking brake mechanism freezes up, the lever cannot move, which does not allow the piston to retract into the caliper. The solution, unfortunately, is replacement of the entire caliper assembly.
To finish up the disc brake service procedure, check the brake fluid level and bleed the brake system. Your customer can now enjoy optimum stopping power.