Sorry long day of eating and getting **** face
any way some of that corrosion is cause by water and or possible battery acid.
The crystalized in the reservoir is because of Dot 3/4 brake fluid is hygroscopic, that is it absorbs moisture from the atmosphere.
If the brake fluid is not changed it will continue to absorb moisture, and eventually it will crystallize and form deposits inside the brake system that can cause the seals to fail and components to seize.
That why you want switch over to Dot 5 and best of all it doesn't eat up the paint.
Note: If I you I flush out brake lines and rebuild caliper and clean out the cross piston passage or just replace it with a new set EBC pads.
Yup your life depends on those back brake work to if the front failure.
Keep in mind the better shape it in the more value it being so as I see it money well spent.