Where are the spots on these trucks that are known for vaccum leaks?

I know I have one, but I cant seem to find it.
Everytime I hit the gas, even slightly my vents go away and switch to defrost, and when my windows are open I can hear a hiss when the tranny downshifts, and the rpms change...

Where should I look?

 

Josh,

This is a common problem, many threads on here about it. What you are looking for isa one way valve. It islocated in the vacuum lines at the firewall on the passenger sideof the truck. These tend to go bad alot.

I am not sure about the hiss you are hearing but you may have multiple leaks.

 

Alright thanks...

I will look into that.

 

Underneath and near the battery
where acid fumes do much damage.

Where the vacuum lines go through the firewall.

Behind the dash in the heating/Air conditioning controls