Delica Canada | Mitsubishi Van Club

A few weeks ago I went to evacuate my A/C system to recharge it (97 L400 with front and rear heat & A/C). I took it to the shop and it was vacuumed for about 15 mins - then sat to see if it would hold. It would not hold vacuum and the needle on the gauge began to creep almost immediately. I had a look at the external lines and hoses. The only place I could see a slight bit of oil was at the connections to the rear evaporator. I removed the evaporator, took it in to get tested (it passed) and I replaced the O-rings at these two connection points.
I took it back last week and it was vacuumed again. It held vacuum a bit better than the time before but the needle on the gauge began to creep again indicating a leak.
I have looked at what I think are all the A/C connections and can't see any oil on them.
I was thinking of replacing all of the o-rings anyway just to ensure that's not the issue.
The A/C guy said I could leave it for the day and they would pressurize it with nitrogen in attempt to find the leak. That sounds costly.
Anyone have any ideas?

Pressurize with nitrogen, leak should be obvious. The pressure relief valves are often stuck open, may be one of them.