Prevents caliper pistons from over-extending during the bleeding process. Simulates the thickness of a brake rotor. Use case: For bleeding Shimano hydraulic disc brakes, specifically BR-R9270, BR-R8170, BR-R7170 calipers. Also useful when transporting your bike without wheels (prevents accidental piston closure). Material: Usually heavy duty plastic. Also known as: Brake pad spacer, bleed block, caliper bleed 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 bleed funnel or syringe, a 7mm wrench for the bleed nipple, clean rags, and isopropyl alcohol.