Its been quite a while since I worked on one and the ones I worked on were sold by Stihl but they probably all have similar parts. This plunger valve is spring loaded, pressure pushes it back. The spring pushing it closed so yeah they corrode up when setting. Not sure about what to use, I've been told there is a pump oil for pre storage but some sort of anti-freeze may work well.

The plunger valve is on the high pressure outlet side of the pump, the main pump body doesn't have to be taken apart.
LMRM; Bob