So after owning a bike with really powerless and squealy rear brake for a year I finally decided to look into the matter yesterday. Turned out the rotor and pads were covered in oil. Without looking at instructions I cleaned the pads and the rotor. The brake went from really bad to useless. Doesn't bother me that much really as I use front brake only.
IDK why I posted this, heads up maybe. In a few years we'll all be laughing to this