Well it's not going to plan
The safety valve end of the pipe is a torturous route to get it in place I have had to position it 5 times now trying different methods of inserting the pipe. I can't seem to get the union to bite properly ....a few times it has kidded me only to jump out.
I've done one of these pipes before and don't recall having trouble with the threads - the biggest problem is access (more so on the Activa due to additional pipework in the area)
I turned my attention to the regulator end of the pipe to satisfy myself that the union thread wasn't the problem. This seems to have needed an enormous amount of pressure on the back of the union for it to thread properly and although it looks as though it's a good fit I would be happier with another couple of full-turns going by the amount of thread remaining. There is no way I could apply the same pressure to the safety valve end so after another failed attempt I removed that end of the pipe and trimmed about 2mm off the end and chamfered the end profile. It seemed as though it was winding on with the most back pressure possible given the access angle but it still popped off
I have 2 thoughts on how to proceed:
Firstly remove the pipe from the safety valve end and trim another 2mm off in the hope that the further the flare is in safety valve the more chance of being able to wind more thread turns on to prevent the flare from popping the union back out, re-insert (sounds easier when I type it

) and pray again that it engages properly.
Secondly and probably final option is to strip out from above (lhm reservoir / ABS controller / gearbox mount / anything else in the way) remove safety valve and hopefully get enough access to connect the pipe/s to the safety valve and then refit it all.
Third (I know there was only 2 options ^) option is to cut new pipe where access is good and reuse the better old union on the new pipe and then repair my brand new pipe with a dodgy connector (I've never had to resort to this bodge on any vehicle so unless someone knows of such a suitable joint system) ...
There is no rush and I would rather have been able to complete the job satisfactorily before I turn my attention to starting the engine .....
Any thoughts
Also this diagram is similar to the one in my Haynes manual but I've yet to see the detailed taper at B on any pipes on the Xantia which would certainly allow the pipe to go on more and improve the stability of the whole connection

- Citroen pipe flare diagram