i've got the same problem with the fiesta i've replaced the disks, calipers, pads, drums, slave cylinders, shoes, flexie hose's, master cylinder and it still won't slow down from 30mph in 300 yards hence the reason the exhaust is bent from when i mounted the curb the only thing i haven't changed is the rear compensator (which i only found it had when i changed the flexie hoses). the hand brake works very well so if nothing else the rears should have locked up. has yours got a rear compensator
p.s. has anyone got a rear compensator for a mk1 fiesta