I'm under the impression that the point of brevets is to be self-sufficient, to do them without sag support, catered rest stops, that sort of thing. Like MP said, you'll need to fend for yourself sometimes.

But the amount of support varies widely and your best bet is to find out from the organizers of the brevet series in which you'll be participating. Just don't expect the level of support you'd likely see in a century/double century ride.