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This guy knows what he is talking about: https://www.youtube.com/watch?v=mXQxLK6z5WA There's also some hints in the video where oil might be coming from.
To summarize, use isopropyl or the specialized WD40 brake cleaner spray. You likely have to disassemble the breaks in order to clean them proper.
~~Using the normal degreaser chemicals for cleaning the chain etc is a no-go, they often have an explicit warning text "keep clear of break pads".~~

This guy knows what he is talking about: https://www.youtube.com/watch?v=mXQxLK6z5WA There's also some hints in the video where oil might be coming from.
To summarize, use isopropyl or the specialized WD40 brake cleaner spray. You likely have to disassemble the breaks in order to clean them proper.
Using the normal degreaser chemicals for cleaning the chain etc is a no-go, they often have an explicit warning text "keep clear of brake pads".