Disc Brake Caliper: Reassembly

Apply clean brake fluid to NEW piston seal, and install in caliper bore groove. Apply clean brake fluid to caliper bore. Coat piston and outside beads of dust boot with clean brake fluid.
Push piston through boot until boot is around bottom (closed end) of piston. See Fig 1. Position piston and boot directly over caliper bore. Spread dust boot over piston as it is installed.
With bead seated in groove, carefully press straight down on piston until it bottoms in caliper bore. DO NOT cock or jam piston in caliper bore. If necessary, use a "C" clamp and a block of wood to bottom piston in caliper bore.

Fig 1: Exploded View Of Caliper (Front Caliper Shown; Rear Is Similar)

Courtesy of FORD MOTOR CO.