Get a vice (or a helping hand),
take the lever blade off, get some long nose grips, clamp down with both hands onto the edge of the piston,
get someone to fill a syringe full of water, don't use air as it compresses and you have less force,
whilst you try pulling the piston out, get someone to push water through with the syringe,
If all fails, I have a silver lever body you have borrow, which has a piston in it already.