Unless you are going to rebuild your old one. You can put the piston on a slightly open vise, with the plunger hanging down.
Put a dowel in the hole the brake pedal pivot goes in to and give the doemerl a wack with a hammer.
If the rubber keeper is not in your new one ( use a flashlight you should be able to see it) and the old one is too hard and /or brittle replacements can be ordered.