The aircon has failed on my 2005 TD4 and the culprit appears to be at least one of the o-rings where the hoses join onto the valve at the bulkhead (there is quite a residue around the union which points to an escape). Now I have an offer of a free re-gas from a friend who runs the local branch of a national crash repair chain so I'd like to give the repair a go myself (as the o-rings are cheap etc) and get the re-gas done. See parts 2 and 3 in the image.

My question is, can I replace the o-rings without having the system gas removed first? I ask this because ideally I'd like to only have one visit to his premises (to not be a pain) and presumably most of it has already vented into the atmosphere anyway, or is it irresponsible and/or dangerous?

No because it would be under pressure when you undo the connection to replace the O-rings, but you check if there is any gas left in the system by removing the cap and press the pin be careful though you may let out any gas that is left in by mistake

Wear goggles !!