188 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. Anti-Squeal Compounds Companies that make gaskets and adhesives for automotive use also produce chemically based anti-squeal compounds, Figure 12-23. Most are designed to be installed on the back of a brake pad before installation, in the same location as an insulator. A few graphite-based compounds are designed to be placed on the friction material itself. Read the directions on the compound before use. Lining Wear Warning Devices A small piece of fl at spring steel called a wear indicator is frequently attached to one of the shoes. As the pad linings wear down to their minimum thickness, the end of the spring contacts the spinning rotor. A high-pitched squeal is produced whenever the brakes are not applied, alerting the driver that it is time for brake service. Operation of a typical wear indicator is depicted in Figure 12-24. Another type of wear indicator consists of an electri- cal lead attached next to or inside the pad, Figure 12-25. When the ignition switch is in the on position, an electrical Rotor Wear indicator Pad rivet Shoe New pad A B Squeal sounds Worn pad Goodheart-Willcox Publisher Figure 12-23. One type of anti-squeal compound. This type is meant to be applied to the back of the pad. Chevrolet Figure 12-24. A spring steel lining wear indicator. A—Pads are new the wear indicator is not able to contact the rotor. B— Pads have worn down, permitting the wear indicator to contact the rotor. A high-pitched squeal sound is being produced. This alerts the driver that brake service is necessary. Bendix Figure 12-25. An electric-type pad wear sensor. A—Pad with the sensor inside the lining and a dashboard warning light. B— Brake pads in position and sensor wire lead harness with connector. When the worn pads allow the sensor to contact the rotor, the electrical circuit is complete. The dashboard light will come on, alerting the driver. Brake warning light Sensor A B To brake warning light Wire harness connectors Sensor wire leads Brake pads