The ability of the Subaru disc brake caliper to use hydraulic pressure to move a piston is how this braking device can provide the squeezing action necessary for the creation of friction. The disc brake caliper fits over the brake rotor in a U shaped configuration. This makes it possible for the brake pads to have the ability to apply friction when this braking component begins to squeeze them against the rotors when the driver of the Subaru applies pressure to the brake pedal. It is the movement of the piston in the Subaru brake caliper that requires a polymer seal to contain the pressurized hydraulic fluid from leaking. The polymer seal around the piston in the brake caliper is the only non-metal component in this braking device. It is also the only part that will wear out in time requiring the entire component to be in need of replacement. This part failure can be identified by the owner of the Subaru when a puddle of brake fluid is found near the wheel. The driver will also notice the brake pedal changing from being firm to softer with less stopping power being applied. The leak happens when the seal in the brake caliper cracks from becoming brittle over time. The replacement of the brake caliper is not very complicated. If you can do a brake job, then you can swap out this braking component. The only addition steps include the disconnecting of the brake hose from the caliper and the bleeding of the brake system of any trapped air that might have entered the system. Once this repair job is completed, the Subaru will be safe to drive on the road again.