Nah, dont grease anything in there.
It all just pushes together.
What i commonly find is wrong with mine, is if you take the plunger out of the piston, there is a clear vinyl plastic seal around the edges of the plunger. Its designed in such a way that water pressure from either side will push it harder against the outside container of the piston.
I usually can revive my polaris by taking a plastic toothbrush, or some other rounded plastic instrument, and running it around the inside of each side of the piston gasket, stretching and forming it back outward.
Ive only had to do that once really well, and the polaris has worked like a champ all summer.
If you give me some more specific information, I can help with a more specific/targeted fix.