Pressure Washer Unloader Valve

[WhoWhatNow]

This unloader valve is from a Simpson pressure washer with a Comet triplex pump. The unit was in a flood and was under water for up to 24 hrs. I have the motor working and have drained the pump oil, rinsed with diesel, and refilled with new pump oil. The pump works but I am not getting high pressure at the wand. I only ran it for a few minutes but there was lots of water coming from the area around the unloader valve. I am thinking the valve is stuck.
Exactly what am I looking at here and how should this work? Should I be able to move the piston? How far apart should I disassemble it to get it working? A new unloader is $130 so I would like to make sure this one is truly a problem/not fixable. I have it soaking in Areo Kroil.

 

[Walkers]

I used this company when wor on my hot water pressure washer. I woman there emailed a little with me and basically talked me through rebuilding my pump and installing and setting a new unloader. Highly recommended.
https://www.pressure-washer-parts.com/

On my regular pressure washer, which is very similar to that, I just cleaned it up, sprayed some lube on the moving parts and exercised them with a rubber mallet to get the moving parts moving again.