Abstract: A pneumatically operated disc brake piston installation tool is disclosed for use in installing a piston into the piston bore of a disc brake caliper housing. The tool comprises a first piston engaging plate and a second plate positionable against the inside surface of an outer shoe bracket of the caliper housing. A cylinder extends from the second plate. A piston shaft is centrally attached to the first plate and extends perpendicularly therefrom, passing slidably through an opening in the second plate into the cylinder. In response to pneumatic pressure, the piston shaft is pushed outwardly against the piston to force the piston back into the piston bore of the caliper housing.