It needs a check valve between the vacuum source and where it hooks up to keep the vacuum from being too low when you accelerate.
Here is a diagram of how it is set up. Stock, there is a vacuum reservoir, but I unhooked mine and it still works fine.
Note that the blue line is a vent. Also, be careful, there are some diagrams floating around (color homemade ones) that don't show the checkvalves correctly.
The vacuum lines are almost 25 years old if they are original. Probably should just replace them while you are messing with it. Regular autoparts store rubber ones are fine.