Undid flange fasteners of newly installed gate valve at IP6 location and used the overhead crane to separate the valve a few inches from the port.  Visual inspection of vacuum seal weld looks normal.  Knife edge imprint in copper gasket indicates that gasket was "cocked".  Installed new gasket and torqued flange metal-to-metal, i.e., no gap around entire periphery.  Also, installed 1.5" valve and adapter to allow pump side of closed valve to be evacuated.