First of all, are you losing fluid from your brake resovoir?
If not then you could have a couple different issues...
1. You still have air in the lines somewhere like above posters mentioned.
2. Master Cylinder seals have went.
If its the second one your car would hobble to one side when you brake because of the design of the seals inside the MC, if you have been driving it that is.
If you are losing fluid, you simply have a leak, source the leak and replace the line.