One of the rear pads was not sliding correctly due to corrosion so the pad has uneven wear and I need to replace the pads/rotors. I went at it today but I was unable to press the caliper piston it at all to make room for the new pads. I was able to get it back together with the old pads for now. Is there a trick to depressing the pistons? Do I need to remove the pressure somehow? I did disconnect the battery before starting. I've replaced brakes on a ton of other cars before and have never had this issue.