My 99 caravan does the exact same thing as what you describe. I have been trying any trying to diagnose the problem. The rear brakes and lines have been completely replaced (and I mean completely) and fronts inspected without finding anything wrong. I can only think of 3 remaining factors;
1 - master cylinder
2 - vacuum assist can
3 - anti-lock brake controller crap (I hate anti-loc) never had any warning lights or anything.
I'll follow this thread as I'm curious and if I find my problem I'll post it too.
Thanks