Originally Posted by Deej101

I know, right? Probably a bit of OCD involved.

At first, it was because I suspected a leak, so wanted to do leak detection, starting with a clean reservoir. Then, it became a curiosity and a quest, because it didn't make sense that it wouldn't clean up.

I've given up on it and just put it back.

I think I've figured out at least why the top is stained as it is: this was probably a replacement (this truck had a front right collision in its history), so the reservoir had probably been sitting around somewhere, upside down, and when the fluid evaporated, the concentrated dye stained the top underside.

Once I have a plausible explanation, my brain lets the issue go.