Prevents the brake caliper pistons from extending too far during the bleeding process. Simulates the thickness of a brake disc. Use case: For bleeding hydraulic Shimano disc brakes, particularly BR-R9270, BR-R8170, and BR-R7170 calipers. Also useful for transporting the bike without wheels, as it prevents accidental closing of the pistons. Material: Typically sturdy plastic. Also known as brake pad spacer, bleeding block, or caliper bleeding spacer. Compatible with Shimano BR-R9270, BR-R8170, BR-R7170, and possibly other Shimano flat-mount calipers. To bleed your brakes, you will also need Shimano mineral oil, a bleeding funnel or syringe, a 7mm wrench for the bleed nipple, clean rags, and isopropyl alcohol.