I have the same problem on my 07 expy and also had the same problem on my 07 sport trac that I got rid of. the sport trac I bought new and I had it back to the dealer 5 or 6 times they bled the brakes, replaced the front calipers, replaced the booster and master cyl, and did a tsb on the brake pedal, nothing helped. finally at 24k miles the rear brakes were worn down and I was about out of time on the warranty so I traded it in, but I thing that it might be something in the brake bias in the ABS unit. I think that we are stuck with the problem. I just bled the brakes on my expy and i dont think that it helped.