Start at the vacuum lines found at the firewall. This is assuming that the other vacuum accessories such as power brakes and shut off work okay.

Apply vacuum to the lines that go through the firewall and see if they will hold vacuum. If I remember correctly, the vac door lock feed line is yellow with a green or purple stripe. If it holds vacuum, then leave it applied and lock the door with the drivers door lock. Roll down all the windows before you start for convenience.

If it does not hold vac when locking the front door, then start tracing the lines and applying vacuum until you find the diaphragm that does not hold vacuum, that is your leaker.

Get a performance parts catalog and they have a vac diagram in them. At least they used to.

Good luck,