There's no single Allen wrench size used for *all* brake work on a 1996 Chevy Blazer. You'll likely need several sizes. The most common sizes you'll probably encounter for caliper bolts and other brake components are
7mm, 10mm, and possibly 13mm. It's highly recommended to have a set of metric Allen wrenches (or better yet, a socket set with Allen heads) that covers this range, as well as other sizes.
Before starting any brake work, consult a repair manual specific to your 1996 Chevy Blazer. This will give you the precise sizes and torque specifications to avoid damaging components. Improper torque can lead to brake failure.
