Brake caliper
Definition
The brake caliper converts hydraulic pressure into clamping force on the pads: a floating (sliding) caliper has one or two pistons on the inboard side and slides on greased guide pins in a carrier, while a fixed caliper has opposed pistons on both sides and does not move. The piston seal doubles as the retraction mechanism, rolling back slightly to release the pads, and a rubber dust boot keeps water out of the bore. Failures are corroded guide pins or dried grease that stop the caliper sliding (one pad worn to metal, the other like new), a pitted piston or torn boot that makes the piston stick (dragging brake, hot wheel, pulling to one side, burning smell, reduced fuel economy), seized bleed nipples and a collapsed flexible hose that holds pressure on. Rear calipers with integrated handbrake mechanisms also seize internally and need to be wound back for pad replacement; electric parking brake calipers must be retracted with a scan tool. There are no direct codes; related symptoms are judder, pulling, premature pad wear, hot wheels and on EPB cars C-codes such as C1A00-series or VAG 01435 when the motor stalls on a seized piston.