Prob have to use some heat on the metal brake pipe nut to remove it if its been on there for years!

Soaking it in WD-40 wont really help i dont think cos if WD-40 can soak into the hose before it is slackened you would think that brake fluid would also manage to come out !!!!

If you dont have any means of heating the nut then prob next best thing to try is hole the flexi hose with a good pair of self grips and use a proper 11mm brake pipe spanner to try removing the nut, could also use self grips on the nut to give a little more purchase.

Hope this helps and good luck.