vent control vac issue

I have a 95 Astro 4.3L V6 and have always had a problem with the defrost, heat, vent settings not working consistently (air flow constantly switches between defrost vents and lower vents based on engine loading) At one point, a shop replaced the entire heater/AC switch control module in the dash - but this did not solve the problem. I replaced the main vac hose coming off the engine intake manifold to the small plastic tank in the engine compartment, with no change. I have had the lower interior dash apart and there are a lot of small plastic vac lines coming off the hearter/AC switch unit - but I am clueless where to start looking for the problem and how to diagnose a vac leak in general other than looking for a cracked line. If anyone has been through this issue and knows the best place to start looking for the problem, I would appreciate it. Also would like to know where the vac line source comes through the firewall into the interior compartment to the heater/AC switch unit.

try a search here in the forum. It seems to me I've read several threads on this problem. I have an older model so the vacuum lines are a bit different. If memory serves, there is a plastic vacuum line that runs up and over the engine at the firewall that is prone to crack.
The hunt n peck approach is the best you'll be able to do. disconnect each end of every hose you can get your hands on, and trace the length of the hoses one by one. You'll find it. A few minutes of patience will save a trip to the mechanic.

It sometimes takes a good ear to find the hissing vacuum hose. Once you do you're golden. On my 4wd jimmy the hose from the transfercase to the vacuum switch had rubbed against the front driveshaft causing a hole. Godd luck!

I have a 97 Astro with the same problem.

I found the plastic tube connected to the vacuum canister near the AC unit. This hose is cracked and broken. I still cannot find the source at the firewall or back of the engine.

Cannot see much there !

I will keep you posted.

Check this thread- it has pictures

http://www.automotiveforums.com/vbulletin/showthread.php?t=406971