vaccum leak question

My 97 f-250 is having brake vaccum issues. I pulled the hose off of the brake booster and run a vaccum test and it seems to be good at about 24. My problem is when I shut the truck off it loses all vaccum in about 4 seconds. Does this sound like a vaccum pump going bad or maybe a leak in the hoses. I inspected the hoses and they seem to be fine. I get the default defrost and brake pedal issues like there is no vaccum upon start. After the vaccum builds up everything is fine until I shut it off again.

There are a whole bunch of places that vacuum could be leaking from.
Throw a mity-vac or other form of manual vacuum pump on the line coming from the vehicles vacuum pump. Pull vacuum and listen for the leak.
Brake booster would be my shot in the dark guess.

Without the engine running you are going to loose the vacuum quite fast, but then again these trucks are getting old and the rubber vacuum lines are getting old and brittle and start to tend to leak. That along with whatever they are going to loosing the seal that they had from the factory.

ok guys thanks for the info. I will try a manual vaccum pump and see if I can hear a leak.