Remove the glove box. At the lower left of the glove box you will see a long manifold with a bunch of vac lines connected (this is called the switchover valve.) Remove the vac lines one at a time and connect an auxiliary vac source and check for leaks.

Five 'll get you ten, at least one of these lines will not hold a vaccum. If I am correct on that, remove the dash and replace ALL the vacuum pods. If one is bad, they all soon will be. Check all of them again at the switchover valve before putting the dash back together.

I suppose that it is fair to warn you that dash removal and replacement is about a 15 hour job.