There is very little in a brake caliper. All that moves are the pistons (apparently called pucks in the US). These are usually hard chrome plated to make them smooth and wear resitant. To seal the pistons in the bore of the caliper is a round rubber seal. This is what you get in the repair kit.
A number of things can make a caliper scrap:
- Corrosion of the piston (other than superfical corrosion on the outer side of the seal)
- Scoring of the bores in the body
- Hamfisted attempted to separate the two halves of the caliper. The seal between the two halves depends on a completely flat and clean mating surface. This is easy to damage or get dirty.
Because of these factors, it is usally wise to buy reconditioned calipers. The cost is very small compared to the importance of properly working brakes.