Thanks, Chris. I don't think I described the spring problem too well, what I meant was that I couldn't get the plunger, with spring or without, all the way down its cylinder. However, by dint of constant hand pressure from on top, I was gradually able to force the plunger down and onto its thread, and it now appears to be working, so it looks as if it was hydraulic back pressure from the oil itself. A new problem emerges, though, namely that the oil drains into the timing chest by gravity, eventually exhausting the tank. As the oil tap has no 'off' position, this seems to be a design flaw; I would have put a three-way tap so as to provide for 'off', hand-pump only, and hand and mechanical pumps. Anyone else got experience of this?