Simon is missing the point here
This is a flare nut spanner, designed especially to tackle the sort of job you are trying to do
Attempting to remove a brake pipe union, especially one which will have been assembled dry by whoever fitted the new front to rear pipework with an open ended spanner is asking for trouble, as you've already found out
You can create your own spanner, by chopping a 3/16" clearance slot into the broach of an 11mm ring spanner with an angle grinder if you wish, however these are cheaply and readily available at most places, even Halfords if you are desperate enough
For example........
LINK
Brian
