Do this;run the truck, (in park only) with your Brake-Booster vacume line
blocked off. See if it still does it. It will help over come your leak do to the
fact the booster draws a lot of vac.
If not, your leak is pretty small and will be hard to find.
Then, start looking at all your lines. Use a squirt-bottle with a little
Dish-washing fluid in it and start wetting down your soft lines around
the manifold and back by the firewall. Have someone rev and then get off the
gas a few times while looking at what you got wet. You should see bubbles if you got a leak.
Also, check under the dash. See if you or a passenger got their feet up under
there where they don't belong and snagged something.