[68c10airstream]

Hi firstc10, I went to auto schooling in the 70's and remember the vac tank well. It's purpose is to check and hold vacuum, because when you step on the gas manifold vacuum drops off, and the vacuum that was in the lines would be bled off into the intake and there would be no vacuum available. 2 lines hook up to the tank, engine supply, and the other line goes to the controls inside the cab. It can be hooked up wrong (backwards), so here is the test. Use 2 vacuum hoses, install them on the tank, put your finger over the open end of one line from the tank, then suck on the other line that is hooked to the tank. When you release the vacuum that you put on the hose, there should still be vacuum present on the hose that your finger is on. Another words you are acting like the engine would with vacuum present, then bled off, but there should still be vacuum remaining on the other line. Good luck, Brian Fuller