it's easy. just stick a piece of wood in the middle of it that won't let the pistons come all the way out and apply air pressure as Ige suggests. They'll pop out most of the time, sometimes pretty quickly (like really fast... take fingers off fast) so make sure the area they will shoot out through is clear of fingers.
if one sticks, you can work it with the others still "kinda" in.
then once they are all pretty much out but still hangin in their bores, remove the wood and finish popping them out.