braking system?

The trouble I am having is with the braking system, the brake pedal goes to the floor when you step on the brake. To solve this problem I have replace front and rear brakes and the master cylinder I have checked the pin in the vacuum booster and bleed all the lines figuring the job was done. The brake pedal is firm when not starting vehicle, but when the vehicle is running it again goes to the floor. I am thinking of replacing the ABS unit. All lines going to the unit seem to be operable but I am not sure. There is no fluid coming out of the bleeder screws in the top of the unit. I know the abs wheel sensors are not working but I just had this vehicle certified like that in late November and all brakes were working fine at that time. They don't seem to be locking up properly when on a test drive. Maybe you can give me a suggestion. Thanks!