Before I leave the thread:

The vacuum modulator is connected to the blue vacuum amplifier on the left side of the firewall. The other connections on the amplifier are a vacuum source, a feed to the VCV on the side of the injection pump, and a feed from the boost port on the side of the manifold. Check for a loose connection, especially on the VCV where it's tough to see. The shutoff and the brakes will usually work, even if there are large vacuum leaks elsewhere in the system. That part of the system is fed by a different port on the pump.

Finally, the vacuum leak can be anywhere under the hood. What you want to do is start at the small vacuum port on the pump, and move forward through each branch checking for vacuum.