what you describe could be what is wrong with the master cylinder. To verify this look behind the dash to see if the fluid is leaking inside the car along the firewall or under the hood on the firewall.
what you describe could also be not enough fluid in the master cylinder and/or air in the system. check your fluid level, then bleed the brake lines first and see if this remedies the problem. You may also have a leak somewhere in the system. Wheel cylinders in the rear are usually the first to go. Before you bleed the system check to see that there are no signs of leakage at all 4 wheels.
good luck with this and if you do not know how to do any or all of these things get help or take it to a place that can help. You do not want to get anything wrong when it comes to your brake system. | 
