Doesn't marr the surface of the piston that way. With new O-rings the pistons will take some effort to install. Removing the center plug. It will seem very stiff as it has an O-ring on it. Just underneath the plug is a spring which holds a check ball into a dimple on a plate inside a pump sleeve.
All parts are necessary.
DO not loose them. The pump sleeve can be removed by pushing it out from inside the front of the OD unit while supporting the piston and cam socket. There is a flat and slot to one outer side of the sleeve. The sleeve must be reinstalled in the same orientation with the slot toward the hole the screen fits into in the bottom of the unit. Remove and replace the O-ring on the sleeve and plug.