"some firm braking to see if it disappears"
Whether on my motorcycle or in my car I take a pride in using my brakes as little as possible, relying upon anticipation and engine braking when available, as advised by advanced driving techniques. Surely it is disingenuous of manufacturers to suggest that enthusiastic brake application is required to eliminate weird brake behaviour.
Maybe I should try an emergency stop on a greasy downhill bend during twilight over fallen leaves in sub-zero conditions, to see if my ABS, ESP, GBH, whatever, actually works!