Heating / cooling system help needed!

[Fastcar]

I have a recurring problem - the ventilation system will only blow air on the floor and through the defrost vent regardless of the vent selector switch position. I know that this is caused by a vacuum leak. My mechanic has replaced the vacuum diverter (cheap plastic $5-10 part) twice, and the system works normally for a while. The system just crapped out again: my mechanic asked a GM mechanic friend and was told that the valve will crack if there's any moisture in the system. How do you get moisture into the vacuum system? My guess is that there's still some other vacuum leak that he hasn't found yet. In addition, under hard acceleration (low vacuum) I'll get the defrost / floor vent problem. Once I get to cruising speed when the system works, it'll blow air out again from the dash vents. Anyone else have this problem, and how did you fix it? Do our cars have a vacuum reservoir, and where is it located? Thanks!

[OreoLt1]

There has to be another vacc. leak or a malfunctioning vacc. check valve. When the system is working properly at idle the mode doors have enough vacc. to hold them in the desired position, when you accelerate vacc. drops. Once it drops to a certain point the mode doors will close due to lack of vacc.

I would follow ALL the vacc. lines from the intake to their destination looking for leaks. If the is a check valve test it, and if still no luck you need to find a shop that has a smoke machine that ingests a detectable vapor/smoke into the vacc. system to find your leak.