you may not have enough fluid in the master, improperly bled lines, a broken line, or a bad master cylinder. get someone who knows brakes to check it out. dont mess with them if your inexperienced, may end badly...
try and use a pressure bleeder and if you cant get your hands on one of those or if that doesnt work try to bleed it from the highest point on the line. I know there is a bleed valve for a reason, but I had a motorcycle that was impossible to bleed unless i cracked a nut at the highest point and bled it from there.