Vacuum Leak location?
 

I believe I have a vacuum leak, but don't know where to look to find it. The only thing I know for sure it is affecting - is the fan when I go up hill or have to accelerate hard. At that point the fan (with the a/c running) stops blowing out the front vents and tries to go out thru the defroster. As soon as I start down hill or step off the gas pedal the air comes out the front vents like it is suppose to. Haven't noticed the problem with the heat going down thru the bottom vents - just the a/c with the front vents. Any help to point me where to look for the leak would be appreciated.

There is a vacuum reservoir that looks like a large Tomato juice can, It might be a good idea to check that can for having a rusted through spot with a possible hole that may be leaking. Let's start by confirming the reservoir is is not losing vacuum.

Check the rubber boot that is on the air intake from the PVC valve. Mine was rotten on the inside, hard to tell if it is bad.