Normally, my diesel westy can sit for weeks and have vacuum in the
brakes when I start it up. Here's the weird thing that's happening...
I still have vacuum when I start up, but within a few hundred feet I
(sometimes) lose it and then get it back after a mile or so. After
that, it's fine until it sits overnight, and then it may or may not
exhibit this symptom.

Any ideas?

Jim