I'm pretty sure I have a vacuum leak. sometimes when I'm accelerating it quits blowing thru the dash vents and goes to the defroster and if I take my foot off the gas pedal for just a second or 2 it returns. I'm under the belief that the defroster is default when vacuum system doesn't work properly.
Mainly I'm just looking for pointers &/or hints where typical leaks may be. I saw one guy said to look at a line by/behind the battery. I saw a square-ish box about the size of a large sponge there, is that a vacuum accumulator of some type?

 

What are you driving? I know on my f250 I had the same thing it was caused by a vacuum leak in my auto locking hubs. When I would switch to 4 high the ac would change to the defrost. Fix the leak fixed the swapping If that is not your case you still have a vacuum leak some where

 

Yes, that's an accumulator back there - but the most common source of leaks are cracked vacuum lines themselves behind the battery. They are small plastic hardlines.

 

Oops, I'm kinda new here, thought my vehicle type showed up with name. It's a 99 f150 4x4 and the hubs are permanently locked.