The BRAC-X Four Port brake caliper is the assembly which houses the brake pads and 4 pistons. The pistons are made of chrome-plated iron. The BRAC-X Four Port brake caliper is a fixed caliper does not move relative to the disc. It uses 2 pairs of pistons to clamp from each side of the disc, and is more complex and expensive. Brac-X Ventilated disc design helps to dissipate the generated heat and is commonly used on the more-heavily-loaded front discs.