Does any one know if it is normal for the ACC switching valves make a hissing noise (noticeable vacuum leak on the side opposite to where the line fits in the unit) when they are working correctly? It appears to be from the five pot (#12) valve on a 87 190e 16V. Thanks.

Tinker

The only hissing you should hear from a working system is the refrigerant passing through the expansion valve into the evaporator.

The vacuum used to move doors and such is captive and should normalize once the action has taken place. You might hear the initial switching but it should go away very quickly (1-2 seconds).