When you bleed the rear wheels, don't build up pressure by pumping the pedal before opening the bleed nipple since the pressure limiting valve will operate and prevent any more fluid passing to the rear wheels. Open the bleed valve first, pump the pedal slowly and hold down, close bleed valve.
Alternatively, try using the 'gravity' method of opening the bleed nipple with a short hose going into a catch bottle. Leave open for a while (10-30 mins - variable), topping up the master cylinder as needed. I did this to my last rebuild and it worked perfectly.
The pressure relief valve is available new 21A1774 according to MiniSpares catalogue. Your brakes should be the same as Mk2 cooper S - single circuit.
http://www.minispares.com/product/Class ... 0to%20shopOverhaul kits were available but one of the seals has to be swaged onto the piston, from memory, and the bores get pitted so a new one is the easiest option.
