190E 2.6, vac drop at heater valve w/ AC on?!

[2manyoldcars]

With temp wheel on hot, AC off, any button pushed, vacuum at heater valve is around zero so valve is open, heat on, normal operation.

With temp wheel at cold, AC off, any button except defrost, vacuum at heater valve around 25 in hg, valve closed, heat off, normal operation.

With AC on, any button pushed, vacuum at heater valves drops to around 5 in hg! Is this normal operation? Will 5 in hg keep valve closed? If there is a leak, any hint on which system/lines to check?