Union on rear brake cylinder seized..

[laurence_13]

Just gone to replace my brake cylinder as it was leaking, went to undo the union nut that according to the guide is 11/13mm... 11's slightly too big and grips but not enough to undo it and 10s far too small.

Thought about undoing the pipe where it joins the flexi bit of hose and threading it through but that's stuck solid too...

Any ideas?

Cheers laurence