longston you have to
have power on both sides of that pressure switch for the compressor to come on. The switch will make up at the proper pressure. You can safely short these leads just long enough to see if the compressor comes on. If the compressor comes on when you bridge it then get the system evacuated and recharged with freon/dye. Find a good independent a/c shop where you can go in and talk with the guy doing the work. After a couple weeks running with the dye any leaks will show up and then you will need to resolve the leakage issues.
The vacuum can get a little more cumbersome because of the limited spaces underneath the dash. You will want to try to get a copy of "Electrical Troubleshooting Manual" for your model if it is available (call the 800 MB number to see if they are available). This manual has really good schematics showing the vacuum and electrical systems on the 300D.
When you say you put a vacuum on the pot to the right of the accelerator pedal I suspect that you are "closing" the defrost vacuum pot which forces air through the dash vents when it is closed off. I got in there and just dug around and repaired leaks/problems as I could get to them. After getting everything working as well as it could I finally just tie-wrapped the defrost vacuum pot in the almost closed position. I want all of my cold air coming through the dash vents.