To add, don't try and use a clamp or plyers on only one side of the piston. Make sure (as someone mentioned above) put something solid over both sides of the piston so the pressure is evenly distributed, basically means all the force is pushing the piston straight in, and not any force to the side.
It's actually really easy to damage a brake piston by clamping it on one side only and trying to force it back in.
You shouldn't need THAT much pressure either, to give you an idea on my motorcycle I can actually do it by hand, without any tools. A car not quite, but if you are really having to put huge force on a clamp you are doing it wrong.