Berlingo Leaks on vacuum, not pressure!?

[Swedal]

This is a 2010 Citroën Berlingo with 5SEL12c compressor. It had zero pressure so I added 120 psi of nitrogen to find the leak. Not 1 psi drop for 20 mins so I began pull a vacuum. Could not get a vacuum and when isolating the vacuum pump I could see the needle return to zero. Back on with nitrogen and no leak! Service port Schrader valves are occupied with my service ports but could it be a third port somewhere? Compressor shaft seal? How will I find a leak only with vacuum?