Could be a bad master cylinder, or a distribution block or a plugged line. Are you not getting any fluid out of the bleed screws or did you crack the line loose? A lot of time the bleed screws plug up and you'll just think there is no fluid there. Pull one all the way out and check, then if no fluid, crack the line loose and check and just keep working your way back till you get fluid. If your master cyl. has been replaced before, some have bleed screws on them and you could see if you have any pressure there also. Put a hose on it with the other end in a plastic bottle, crack it open and see if you get fluid when someone steps on the brake. Don't go all the way to the floor with the brake pedal or you'll trip the piston in the distribution block and your brake failure light will come on in the dash.
Steve
PS. Usually when it's the master cyl., none of the brakes will work or it leaks.