I've never appreciated the value of this pressurization feature in our LIVP planes.  The recommendation is to wire the Dump Valve to the airspeed switch according to the description below, but I did not, and see no value in doing so.  In fact, if airspeed is artifically reduced from pitot ice or some failed pitot tubing at one of the couplings, you will still want to maintain pressurization as the engine continues to hum along int he flight levels.

I figure that the turbos provide pressurized air, and when the throttle and MAP drop below pressurization levels (let's say 28", depending on altitude) the cabin air will vent quickly either through porous sources or back out through the mixing box and back into the upper deck.

Either way, when you reduce throttle to something below ambient pressure, like when you're landing, the cabin will normalize with the outside air pressure long before you arrive abeam the numbers on downwind.  This action does not require an airspeed switch to then open the Duke's Valve cabin dump.

To restate my observation, in our planes, the way they are built, there is never a time when you can or will hold pressurization on the ground after you land, however you set up this auto cabin dump feature (in my case, totally disabled).  Other planes have one way pressurization valves, but we do not.  The mixing box input hole (bringing pressurized turbo air in) is the biggest 1" hole in the cabin to vent air back into the upper deck when cabin pressure exceeds mainfold pressure.

Jeff L

LIVP