You need to remove the front
Brake Caliper, the brake caliper piston, the caliper dust boot seal, the piston seal, the bleeder valve and cap, clean the brake caliper piston bore and seal counterbore and the caliper piston, dry the caliper piston bore and counterbore and the piston, inspect the caliper bore, and inspect the caliper piston for cracks, scoring, and/or damage to the finished surface area.