Hydraulic jack fluid.

hemi71x said

I'v got a lot of old pistons with that mark on the bottom of them, too.
Never gave it a thought.
Most likely just a machining marking from the company that manufactured them in the beginning.

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all 4 were the same height. I saw that on some calipers there are 2 longer and 2 shorter. I wonder if the mark is an indication of that? I thought I saw that pistons with a round grove on the face that presses on the pad was the indication of long vs short?