Brake Disc Pads problems of the 2011 Ford F-150

I was backing out of my driveway and felt the brake pedal get soft like I had no power booster. The truck was very difficult to stop. I had to pump the brake a few times before the pedal stiuffened up a bit. This has happened everyday since, but only in reverse and only if I go in reverse 1st. The pedal seems a little softer than normal going forward but not near as bad as in reverse. I replaced the front brakes on 7/25/2015 (rotors and pads- high end replacements) and that did not help the problem. I just replaced the rear brakes on 8/15/2015 (rotors and pads- again better than oem) and I can still tell there is something not right when I back up as a first action. The pedal still needs to be pumped a couple of times before it feels better.

No brakes when the vehicle first starts up. Start vehicle, shift into gear, pedal does not engage unless it is stomped on and even then its difficult to stop vehicle. I have taken this to dealership where purchased. First time dealer replaced front brake pads with a modified pad and recalibrated the abs module. Second time they replaced brake vacuum pump and vacuum line. Third time dealer replaced vacuum line. All of these failed to solve problem, dealer contacted Ford tech line and Ford engineers told tthe dealer that it was a characteristic of the truck and could not be fixed we called Ford customer service and they opened a case. We recieved a callback from a Ford regional customer service manager and was told if this concerned us that we should just sell the vehicle and buy a new Ford since we have the z plan. There are no warning lights that alert you to this happening! I can not longer even drive the truck I bought because I do not have the strength it takes to stomp the pedal down when this happens. Im so concerned that this truck could hurt someone. Im a 58 year old women and Ford does not even care to resolve this serious safety issue.