Ford Ranger Brake Problems?

I have a 99 ford ranger 4x4 w/ abs.. The brake peddle goes to the floor. AFTER this started happening I went ahead and changed the brakes wheel cylinders calipers drums and rotors all the way around, and bled the system throughly there is no air in the system anywhere and the back brakes are adjusted correctly, tight but not too tight. With out the engine running the brakes feel great, but as soon as you start it up the feel like crap and go to the floor. Like I said this started befor I did anything to the brakes, could this be the abs system?? andy ideas would be appreciated thanx