Did you have this problem before you did your brakes? If so it could be a master cylinder or maybe a leak somewhere else in the system. If you had to pump the brakes to get them to grab only after the repair then you probably just need to bleed them better. Have someone pump up the pedal and hold it down, then open the bleeder and close it (do not have your pedal pumper let up on the pedal until the bleeder is closed) repeat on all for corners of the vehicle until only fluid and no air comes out.......make sure you keep topping off the master cyl as you do this. Good luck!