Hi Kenenth,
When Citroen build the car they use a type of tubing nut which is mild steel, 8mm across the flats and is "hollow" for the depth of the hex part of the nut. The steel/steel interface between the tubing nut and pipe is where the majority of the corrosion takes place. The "hollow" provides a nice pocket for moisture and salt to accumulate in order to promote the rusting process. In addition, the "hollow" part of the nut is just behind the hex where, of course, all of the forces are exerted when undoing the nut with a spanner or ring key. A cynical person may come to the conclusion that Citroen intended these to be scrapped entirely once rusted.
There are only two methods I know of to remove the tubing nuts from the Tee-piece:
1. Cut the remainder of the pipe off just above the tubing nut (the pipe is scrap anyway). Now you have access to put a socket over the end of the tubing nut. Use a SIX-pointed socket on the nut and NOT a 12-pointed which are the more usual type. The 12-poters don't have enough contact area and will simply round the corners. Be warned.
Or
2. Cut the remainder of the pipe off just above the tubing nut (the pipe is scrap anyway). Now heat the tubing nut with oxy-acetylene until cherry red (protecting everything else around the joint from the flame). Using a pair of mole grips undo the tubing nut from the tee-piece. It usually loosens no problem once heated. The tubing nut is so flimsy that it is possible to heat it up until glowing red without heating the tee-piece up too much and destroying the seals in the adjacent connections.
Only in a very few number of occasions have I found that I can remove a tubing nut with the pipe intact using the 6-pointed ring spanner with the gap in the ring. MOstly however, even when that special spanner gets the tubing nut rotating in the tee-piece, the rust holds the pipe firmly to the tubing nut and if you keep rotating the tubing nut you simply twist and then shear the tubing.
Whilst its not my intention to try to scare you its worth knowing what to expect before you start. Forewarened is forearmed as the old cliche goes.
Best of luck.
Cheers
noz