One of the parts of changing hydro brake pads is pushing the pistons back. I usually jam a cone wrench in, but never felt right about it.
Well the Park Tool PP1.2 does everything you’d want it to do. Really it is cheap and makes pad changes easy. Just go and buy one.
Read the detaild here: https://www.parktool.com/product/hydraulic-brake-piston-press-pp-1-2
The unique, wedge-shaped PP-1.2 slides into a hydraulic disc brake caliper to preset the piston when replacing pads, aligning pads, and performing other service procedures. Makes hydraulic disc brake maintenance and repair faster and easier.