i suspect you have a caliper or brake drum piston thingy that sprung a leak. take a look under your car, at the backs of the tires, and see if you see any wet stuff on one.
you may also have a blown master cylinder, if this is the case, you should see wet around the unit. i would check the wheels though, something like that happened on our Chrysler E-class. the brakes worked fine at first, and then would drop after you stopped and everything. this was also accompanied by the brake light. is this true?