Does it sink down if you apply a certain amount of pressure? If so, let it keep sinking and see if it gets to a point where it will build up pressure. You can do it while it's stopped. My Dad's 03 does this and it's a bad valve in the ABS module that causes it. Brakes work fine otherwise, just kind of scares someone who jumps in and drives it having never driven it before lol. It almost feels like a bad mastercylinder for a little bit, then catches again lower to the floor (we put in a new master cylinder thinking that was the problem... nothing changed).