Automotive system leaks on vacuum but not on 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?

[RANGER1]

Got to be shaft seal.
Maybe to prove you would have to remove compressor, blank connections, then pressure test, if ok pull vacuum.
A longer pressure test could also help, the latest thing is with helium in nitrogen to find micro leaks.
Shaft seals are not that expensive, so consider changing it anyway.

Just to add, I do not work on this equipment, but sometimes if you are concerned about something, just do it (referring to shaft seal).
Refrigerant leaked out somewhere & seal would be most probable.

[Swedal]

Thank you. I also suspect the shaft seal but I'd rather hoped for something more simple like a vacuum leaking Schrader valve.

[cadwaladr]

I put dye in hard to find leaks on car ac works eveytime

[charlie patt]

I've had enough front seals leak but not to the point it would not pull a vacuum
I did have a pressure blow of valve do the same as you have above held a hp test for then would not pull a vac at all I found the valve on the back of the pump leaking by
like cad says dye it if you want......... to check front seal poor a little 68 oil by the rear of crank pulley it provides a temporary seal

[Swedal]

I'll try the oil trick and see if it will pull a better vacuum. Thanks for the tip! Can not find replacement for blow of Valve though.