I always got the impression that you shouldn't really touch the air-con system as it's a specialist bit of kit
(Refrigeration unit and nasty chemicals inside)
my mum's aircon (on her K12) has never worked right...
she had it "Re-gassed" at £50 a pop, and it worked for almost a month before failing again
...she took it back to check for leaks and none were found so they just topped it up again...for another £50 <_< ...and it worked...for about a month...
We think it needs a new compressor...which none of our local garages can seem to source so we have to take it to Nissan
....gonna cost around £400 for parts and labour...
although one of the garages that tried to get the part for us quoted £300...so £400 don't sound that bad?... *Facepalm*
...so glad there's never been an issue with mine... ^_^
I
hope your simply needs re-gassing... (K11 right?) ...could just be getting old?