I noticed that the pistons on my hydraulic brakes (Shimano M365) leave traces of oil on the back of the pads. Is that ok or it is considered as a bad leak?
The quantity of oil is barely noticeable but it's there. The pads are not contaminated at all.
I should note that the system needs a proper bleed.
Edit : I forgot to mention that the system features an "automatic compensation" for brake pad wear. I am guessing it just lets a little bit of air inside when the pistons extend too much, which could explain the leakage.