If the '79 240 uses the same vacuum colors and approximate configuration, you should have a brown (or brown + stripe) going from a distributor (rubber T-thing) into the firewall where it connects to the ignition switch, then a brown (or stripe) line coming back out and going to the shutoff.

You could plug everything on the distributor except the source of vacuum (from near the brake line) and the brown line. If it shuts off, add a circuit like the yellow line and then see what happens. It's trial and error from there to isolate the specific component that's leaking.