There is a hard plastic vacuum line that goes from the intake manifold to a T over the battery. from the T one leg goes inside to the control switch & the other goes to the vacuum reservoir under the battery. Usually it comes off at the manifold or reservoir or the plastic line itself cracks. Check the rubber fittings at the ends first. If the line is cracked it can be hard to see. You can test this by going behind the control panel & finding the vacuum lines, one should have vacuum when the engine is running. I think it is the black one if I recall.