Caliper Installation Note

WARNING: This page is about a different car, the 2017 Mazda 3. However, it is still accessible from the selected car via links, so may be relevant.

Clean the exposed area of the piston.
Push the piston in using the commercially available disc brake expand tool.

Fig 1: Pushing Piston Using Disc Brake Expand Tool
Courtesy of MAZDA MOTORS CORP.
Install the caliper.