I have stripped the interior out of my car for restoration and am trying the get the a/c complete while the dash is apart...The manual calls for a vacuum check valve I believe located in the engine bay....what is that and what does it do? I have no vacuum lines currently for the a/c in the engine bay....also I noticed when I removed the radio speaker there is a diaphram (shaped like the vacuum advance) behind the instrument cluster gauges with a switch that has two brown wires connected to it and the back of the diaphram looks to have a nipple to attach a vacuum line or something....what is that for and should there be a line attached to it?????