Vacuum loss on acceleration

I have a 2001 Ford Windstar that has developed a vacuum leak or something similar. As I climb a grade, I am in Arizona, the A/C defaults to the defroster and I get a gasoline smell inside the van. After I crest the grade the fuel smell lingers but the A/C goes back to normal. I am looking for a good vacuum routing diagram or insider information from someone who's experienced the same symptoms as I have described here. There's something I am missing, Anyone know what tree I should be barking up?

Take a look here:
http://www.automotiveforums.com/t1050429-a_c_switches_vents_on_its_own.html

It might be the check valve between the manifold and vacuum reservoir.

I find a hand vacuum pump (Miti-Vac) indispensable when hunting for vacuum hose and system leaks under hood.