My 96 GTS is having vacuum issues with the HVAC system. The air only blows through the defrost vents regardless of vent setting on the HVAC dials. This leads me to believe none of my vacuum actuators are working and the system is not getting proper vacuum. The shop manual says to first check the vacuum check valve when diagnosing vacuum issues. It says the valve is locate on the vacuum line coming off the intake manifold on the passenger side of the car. This line is easy enough to find and I can see where it runs through the firewall. It looks like there is maybe a check valve at that firewall, but you have to access it from inside (behind the instrument panel) to test and replace it. Is this the case?

I've read a few threads about people battling with low vacuum during hard acceleration and their vent doors shutting. Some people have fixed the problem by adding a check valve to the vacuum line coming off the intake manifold. This confused me. Since they system (at least what I read in then manual) should already have a check valve. So, why do people need to add one? I'm wondering if people have a bad check valve and are installing a second one instead of replacing the faulty valve.

Bottom line, I want to get my system working. I'm afraid I'm going to have to pull the dash apart to get to the problem, but want to make sure. If anyone has pulled the dash apart, how hard was it? Anything I should be careful about? Any good tricks? I'm always worried about breaking tabs and fasteners when doing interior work. Especially on a car with old plastic.

Thanks for the help!