There's a couple of ways of approaching air leak diagnostics.
The quick check would be jack up the corner that sags the most and let the strut drop down and lift back up the strut again to see if you can hear air leak out. You can try spraying the strut and pipe work with soapy water but results vary.
The passenger side has the valve block tucked up in the arch and it has been seen a few times to have leaking connections so worth spraying it too.
If nothing jumps out at you you could use a valve to isolate the strut to see if it drops over time.
See the reply from Jef in the link to see components and process.
http://forums.ross-tech.com/showthre...eding-Charging