I'm wondering if this is an example of galvanic corrosion. The brass has turned blue near the steel tank and near the stainless (I think) one way check valve.
Galvanic Corrosion needs three things to begin:
1) Two different metals
2) They have to be touching
3) Moisture present
There is a whole science behind understanding it.
Therefor, I take it seriously when doing auto & home repairs.
There is also a chart showing how similar (or different) metals are to each other / affecting their rate of galvanic corrosion.
One example: FORD attaching aluminum panels onto steel frames.
They must use a spacer as a separator.
I noticed you use a cycle stop valve. Have you ever been to the Terry Love plumbing forum? I asked basic questions over there and the minute they find out I have a monodrive they blame everything on that and they try to sell me a cycle stop valve LOL