1994 Cadillac Seville Brake Pedal Goes to the floor

The brake pedal of our 94 Cadillac Seville STS goes to the floor with the engine running. I have replaced the master cylinder and have not noticed any leaks at the brake cylinders. If i pump the brakes, it will build up pressure, or if i am sitting still and i spin the tires (it is winter here now) and i and hit the brakes right after, it works fine. I am wondering if there may be a problem with the vacuum booster, or ABS componants? Other than bleeding the brake lines again, I can't think of anything else.