I know its an age old thread, but it might come in useful for someone.
The calipers are sticking, so the gentle pressure you're exerting wont move the calipers, then all of a sudden the brakes are on. Sometimes its just jerky to drive, other times it's an emergency stop when you least expect it.
Service exchange calipers are expensive, service kits are about 25 quid. Usually the back ones are to blame because of the hand brake and boxes spend more time parked up than ford originally planned.
Jim
June 2015