Hydraulic jack fluid.
I came, I saw, I kicked its ***!
Dana 4 Caliper 1
Piston on the right was the stuck one.
Finally got it to turn, then hydrologic pressure and twisting.
The one on the left is the other from the same 1/2.
Rust in the usual place.
The other 2 pistons are really good but I'll probably replace all 4
