i have a 99 B1500 Conversion Van with the 5.2ltr and when i accelerate the air flow diverts from the dash vents (sounds like it's going to the defrost vents but i can't feel any flow there). it's clearly a vacuum related problem, but don't have any idea where to look for a leak. like most recent stuff there's so many hoses and lines it looks like a couple of "hoovers" have climbed into the engine well and had little uns.
any tips (especially with pics) would be greatly appreciated.

 

after checking top to bottom i found the culprit. elbow connection going into the top of the vacuum reservior was cracked. i replaced it and the air flow is back where it should be, also fixed the cruise control at the same time (i never realized that it was also a vacuum related problem). here are some photos in case someone else is having these problems and want to check this spot.

 

Hope this is helpful as well, but I already knew there was an issue with these vacuum lines, so after I found mine had "unplugged" itself from the tank, I also found the hard plastic line had started chipping/cracking away at one of the bends (after testing the line after the aforementioned tank). Of course a proper replacement cannot be readily found, but I used a rubber vacuum hose piece to "extend" the area that was cracking. Works fine. Glad you resolved it.