Took me a while to find the leak, it's not on the thick single pipe (with inner pipe) section, it's in an area where the single pipe has split into two pipes just a few inches before it enters the rear evaporator, the leak is on a short tightly curved section of the smaller diameter higher pressure pipe. I did order a couple of AC connectors as Bri suggested but they'll be ages before they arrive (from Singapore, just a couple of quid including shipping) so I cut my Halfords recharge kit pipe in half and attached the bit with the AC low pressure connector to a tyre valve, used compressed air to pressurise the system as Gllbert suggested. At 160 psi there was only a very slight weep of green fluorescent stuff from the narrow pipe but when I ran the AC compressor the weep turned into quite a spray. Seems the pipe has corroded / been damaged by eletrolysis from touching a metal panel while damp. On the vast majority of this model vehicle the front to rear AC pipe is one long section, on mine it comprises 3 sections. The 3 section is apparently only ever an aftermarket replacement part, to replace the original single pipe like for like the rear subframe has to be lowered but that isn't necessary with the 3 section aftermarket replacement. Disconnecting the rearmost section (with the leak) from the evaporator was a pain, the single M6 bolt that secures both HP and LP pipes had also suffered corrosion and snapped in half. Still, I managed to disconnect the pipes, then tried putting a couple of nuts on the broken bolt to torque up against each other to try to unscrew the bolt again... snapped again. I managed to drill the bolt out and re-tap the threads, so now I'm hoping I haven't damaged the evaporator by drilling.
With the rear section off I was able to better clean it up, which is when I found just how corroded it was. Still there were only a couple of little pin-prick holes so I set about trying to braze it over and thicken it up. At first the results looked really promising.. and I should probably have left it at that but I didn't! I realised that although it looked repaired I hadn't 'scratched it in' so had a go at reheating and scratching in... which is when the massive holes appeared lol! Then I tried cutting the corroded section out and brazing a bit of tightly bent 8mm copper pipe in it's place using 8mm internal diameter pipe nuts at each end rather than trying to braze copper to aluminium end on. No luck with that either and at this point it was 10pm and time I packed in.
I now have a few options - 1 drop the rear suspension and fit a full length pipe from a scappers, 2. try to source a second hand or new rear section for my 3 section setup, 3. try again to repair this pipe... Might as well try 3 first, won't hurt to try. But I'll try another tack using aluminium aircon pipe cut from a car that I'm going to scrap. One of the problems with 3 is the shape and length of the pipe, I'll have to really tightly bend the scrap AC pipe and don't know whether to still go with the pipe nut type idea or just try to butt braze the pipes together. I imagine butt brazing is extremely difficult!
I do wonder if brazing was a bad idea... If I could have used something like that two-part metal repair stuff instead, perhaps coated with a layer of TigerSeal and Jclips over the TigerSeal after it had set.
I'll add some pics when I get time.