There is a small check valve on the vacuum pump hose, under the plenum, to prevent the pump from trying to pull a vacuum while the engine is stopped. Sometimes this valve goes bad. Try disconnecting to line at the fitting where it goes under the plenum, drivers side, and place your finger over the line (non plenum side) and see if the pump stops running. If so, you can add a small check valve to the line, in series, and prevent having to pull the plenum.
