master cylinder isn't up to the job, blanking the rears wont help as there is 2 pistons in the master and blanking the rears just means that 1 isn't doing anything, the other is still trying to fill 2 bigger calipers, get a mondeo master cylinder,
i've had the same prob with a fiesta turbo, couldn't get a decent peddle after we upgraded the front brakes, bigger master and all was well again