The only thing keeping the brake fluid inside your caliper is some light pressure of the edge of the rubber seal against the piston. The piston should be smooth and as clean as possible at all times. Any imperfection can cause leaks. Any visible damage is life-threatening.
To remove and install pistons I recommend getting horse-shoe pliers or a similar tool that only exerts pressure on the inside of the piston. You should still protect it with a thick cloth or rubber strip. Never touch the outside of the piston with anything rougher than a soft brush.