Possibly master cylinder. Did you remove the rubber valve and spring out of the rear brake line at the master cylinder? You need to remove the line, and then pull out the conical bit with a hole down the centre, and behind that the rubber/spring valve is found.

If that doesn't work, I'd try another master cylinder off a 180B/240K etc..If it still didn't work, then I'd go and find a master cylinder that came off a 4 wheel disc model car.

Chris.