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Tried to open bleed screw on rear caliper to bleed the brakes, 8mm spanner not tight enough on nut will not turn, 7mm way to small! Have i rounded off the nut or do i need a special tool to open the screw?

It is a 7 or 8mm. Check that your tools are not crappy. I saw a tool set once that had a 10.5mm socket that was labeled a 10mm . (wouldn't fit a 10mm or 11mm nut) All the other tool sets fit the same nuts just fine.
No kidding...