I would have thought it impossible to crack a piston while pushing against brake pads but I guess if you try hard enough you can indeed do that. I pretty much ALWAYS use the Park tool and press against the old pads. If that doesn't work I remove the pads and carefully push on the piston w/ the box end of a small combination wrench. I've have not ever cracked a single piston doing this and I work on a metric **** ton of disc brakes.