Photos are of the ABS sensor space, one sensor (wiped the other off before thinking about it) ~ there's not really a lot of Obvious goo in the drums, but enough to get me investigating.
Drivers side:
Drivers side sensor:
Passenger side:
Pulled the breather, it moved freely when I pushed on it but when I introduced cleaner, it didn't pass. Pulled it apart (already had a replacement) and it looked like the rubber seal had glued itself to the opening.
I'm afraid I know the answer, just want confirmation.