I betcha the tool he's using is the little cube with different pin patterns on each side. That cube is only capable of rotating the piston, but it needs to be compressed at the same time in order to make the piston go back.
I've done it by putting the cube on a socket and short extension, then putting a big woodworker's clamp around the entire deal (caliper, tool+extension+ratchet) then turn a little, squeeze a little, turn a little, squeeze a little. Awkward, but it got the job done.