A bleed block, or piston spacer as it is sometimes known, is an object designed to fit snugly between the pistons of your brake caliper (once in the reset position), and its job is to prevent them from moving outwards during the bleed process.
There are certain criteria a bleed block has to meet in order for it to work. It must:
- Fit snugly between the pistons of your brake caliper,
- Be made of a non-compressible material (wood, thick plastic),
- Be a non-marring material - not metal, as this will cause damage to the surface of your pistons.
Here is an example of a bleed block being correctly placed between the caliper pistons prior to bleeding an Avid brake system.
If you don't have a bleed block don't panic, follow our tutorial on how to make your own.