Near the vacuum pump is the vacuum distribution manifold. All the lines terminate there. Some actuator or line is leaking. There are more actuators than you might think; like the one that locks the gas tank door. The hard lines get brittle and crack, Rubber coupler pieces rot and leak. Actuators leak. Get a hand vacuum pump and check each line at the manifold for leak down. The system has several check valves and they are known to fail from time to time as well. I'm sorry I don't have a vacuum diagram, but perhaps someone who does can post it.

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