On the main vacuum line to the brake vacuum booster, near the rubber grommet at the first firewall, there is a small vacuum line the attaches to the built-in fitting on the main line. This is where the shut-off system gets it's vacuum to power itself. I would check here to see if it is broken off, and/or getting proper vacuum. Be careful, this fitting is 21 years old and is likely to be brittle and break off easily. Ask me how I know.