I've heard some rear calipers are 'upside down' i.e. bleed nipple at the bottom rather than the top, try em the other way round

Oh and just to clarify, you want the late MK3 calipers as the early ones are the same as MK2. The way you tell is you check for a handbrake spring, if you can see one they're the old type. The ones you want have the spring inside and so dont sieze

The handbrake cables changed on the MK2 around 87-88ish, you'll need the late type. The h/brake lever also changed about this time, I've heard some say the late cables are a straight swap, and some say you can't fit em unless you change the h/brake lever as well!