300tdi air con leaking?

i've recently got myself a 96 discovery after selling my series, i got it cheap but i have had to to do a few things to it the one thing that has baffled is the air con, before anyone says let a garage look at it i'd like you to know i trained on truck air con admittedly i havent worked on air con for a long time and my brother-in-law is a master tech so trained on car air con. when i got it the system was empty so we vacuum tested it it held vacuum with no noticable drop, we them charged the system with 10bar of nitrogen which it held with no drop, so then we chraged the system , lovely and cold for about 2 days then back to no air con. when we rechecked the system no gas? there are no leaks showing up with uv no oil stains on anything so with out changing everything in the system we're abit stumped so i thought i'd ask and see if anyone else has had similar problems or if theres a common part which dies in discovery air con any help would be great