sport track ford 2001 brake easing to floor

I have 90K miles on this Sport Trak. When I step on the brake I get what
feels like a good pedal until a few seconds then it eases to the floor about
3/4 of the way. (yes I wrote of this a while back) The problem was not
consistent but now after flushing all the fluid it seems its consistent now.
I have new fluid all the way through the system and while I have it all
apart , I am changing the front pads. Its disk on the front only. I see no
leaks and suspect either the master cylinder or the booster but this is one
area I cannot distinguish between the two for this problem. Any help is
appreciated.