Abstract: A disc brake caliper piston diagnostic gauge for the in situ testing and monitoring of a caliper piston which consists of a dial gauge having an externally threaded probe stem housing which extends outwardly therefrom into close proximity to a caliper piston when the gauge is in its operative position on a caliper housing. An anchor plate is threadably mounted upon the stem housing so as to be in abutting engagement with the inside surface of the outboard caliper housing when the gauge is in its operative position upon a caliper housing. A bias spring assembly is provided in threadable engagement with the stem housing so as to be adapted for selective operative engagement with a caliper piston. A contact indicator probe is provided which extends from the dial gauge through the stem housing member into operative engagement with the caliper piston so as to monitor the movement of the caliper piston and to translate the movement of the caliper piston into a visual indication on the dial gauge.