I use compressed air to remove the pistons. That way, the outside of the pistons won't be marred or damage by using a tool. And I can save some $$ by not buying the tool - I'm retired and on a fixed income. First place a thin wooden stick across the pistons to protect the end of the pistons.
I know a guy who had some pistons frozen in the caliper. Compressed air wouldn't budge them. He removed the caliper, removed the pads, and pumped the brake lever to use hydraulic pressure to force the pistons out. Don't know how he managed to get the last piston out, tho. Maybe it was out far enough so some compressed air could finish the job. Or by using two thicknesses of wooden sticks could block the piston that was wasn't as tight to get the other piston out fully, first. You get the idea.
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