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, especially RS505 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, caliper bleeding spacer. Compatible with Shimano BR-RS505 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.