If the master cylinder is leaking internally, it'll act like you don't have brakes. Fluidsome fluid pushes past the piston rather than pushing fluid through the brake lines (and then to the calipers).
If you're not losing fluid, I'd suggest the brake master cylinder. Easiest way to test it is to start the car and press firmly - but not quickly - on the pedal, and hold pressure. If it's bad, the pedal will gradually go to the floor.
(Note that this could also be caused by a leak elsewhere, but you'd be able to tell by fluid present on the calipers, ground or brake lines.)