Trace it back. It the system uses intake manifold vacuum to control/actuate the operation of the various diaphragm valves. The source of the vacuum varies:
The hose can be either connected directly to a fitting tapped into the manifold (frequently it shares the fitting that the transmission modulator tubing connects to):
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Or it can be attached to a spare unported nipple on the backside of the carburetor:
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As far as routing, starting from the reservoir;
The hose should run behind the wiper motor then through the black plastic tunnel on the firewall above the master cylinder. Somewhere (after emerging from the tunnel but before heading down to the manifold) the branch line to the reservoir should be connected to a tee. One leg of the tee will continue along the firewall and then enter the cab through a rubber grommet over by the evaporator housing. That is the hose that connects to the control head on the dash.
The other hose that is connected to the tee is the one that leads to the manifold/carb vacuum source.
That section of hose should have a check valve inserted at some point. The check valve prevents the reserve vacuum (stored in the reservoir) from being lost when intake manifold vacuum is unavailable. It should be oriented so that flow is allowed into the carburetor/manifold. Note the direction of the arrow in the image above.
