The cracked rubber boot on this heavy-duty truck caliper allowed the caliper piston to seize, which caused the entire brake assembly to overheat under severe braking conditions.
Most disc brake calipers have only four basic parts: the caliper housing, caliper piston, piston seal, piston boot and mounting hardware. Disc brake calipers are either of floating or fixed designs. Most floating calipers are single-piston units designed for light-duty applications. Special anchor bolts are used to not only fasten the caliper to the steering knuckle, but also to allow the caliper to slide horizontally to center the pads on the brake rotor and compensate for pad wear. The mounting hardware, which usually consists of lubricated o-rings or plastic sleeves, tends to reduce noise, vibration and harshness (NVH) complaints by insulating the caliper from its mount. Although fixed calipers aren’t generally used in many OEM applications, they are used in performance applications and in older vehicles. In contrast to the floating caliper, fixed calipers are bolted solidly to the steering knuckle and use two or more opposing pistons to center the brake pads on the disc brake rotor and compensate for pad wear.
The disc brake caliper uses hydraulic pressure generated by the master cylinder to press the disc brake pads against the brake rotor. The caliper piston is suspended by a square-cut o-ring seal that fits into a groove cut into the caliper bore. This square-cut seal is designed to retract the piston a few thousandths of an inch to reduce brake drag. The caliper compensates for brake pad wear by slowly advancing the piston through the square-cut seal. As the piston advances, brake fluid is used to fill the void created by the advancing piston. For this reason, the disc brake reservoir is much larger than the drum brake reservoir on many older dual master cylinders.
Caliper Failure Points
Hidden perforations in the piston boot are the most common cause of caliper failures. Once the piston boot fails, dirt and water accumulate around the piston and piston seal, which allows the piston to corrode against the caliper bore. This corrosion causes the piston to stick in the bore, which results in symptoms that usually reveal themselves as a pull in the steering wheel or a pulling sensation when the brakes are first applied. A sticking caliper is also indicated when a routine brake inspection reveals brake pads that appear to wear unevenly from side-to-side. When new brake pads are installed, the piston is compressed into the caliper bore. If the piston is corroded, the piston seal is scored and begins leaking fluid. In any case, the above reasons are why many professional brake installers insist on replacing calipers in pairs when a brake pad replacement is done.
Although disc brake caliper warranties are rare, they do happen. If the piston seal is leaking, the caliper boot will be extended and will exhibit fluid seepage. If the complaint is a sticking caliper, inspect the caliper boot for perforations that might allow water to corrode the caliper bore. Other causes of a sticking caliper might include corroded brake pad mounting surfaces in the steering knuckle, incorrect installation of the brake pad anti-rattle clips or the lack of lubrication on the caliper anchor bolts.
Improper installation also heads the list of warranty complaints. In many cases, the technician switches the left and right-hand axle locations of the calipers, which causes the brake bleed screws to be located at the bottoms of the calipers. This error prevents the air from being purged from the caliper bores. Another common error is re-using the old copper washers that seal the brake hose to the caliper. In some cases, the washer will leak brake fluid, in other cases, it will allow air to enter the brake hose and caliper.