any auto parts store has a tool that can retract the piston back

 

I used one of those cube deals and ground off a bit of metal on the inside of two protruding nubs so it would fit. Worked fine, but took more effort because it didn't fit perfectly. It wouldn't work with needle nose for me, couldn't apply enough force, so the cube worked best for me, and a heck of a lot cheaper than the dedicated tool.