I am sure some of the whizzier kids on here will provide you with an exact answer, but for starters - I am assuming you are positive there are no leaks anywhere on the system? Are there any calipers sticking open?. If all of those are good and you are certain that the Master Cylinder is good then I might suspect the servo. That said I think the symptoms may also point to rear calipers not being wound in when the pads were last changed - If you don't wind them in then you might get a similar problem. Good luck!!