So I, like many others, had the issue where:
-my max AC blows out of the defrost vents at speed
-the actuator behind the glovebox never opened for he recirculating air
-AC is cool at idle but craps out at speed

Most likely a vacuum leak, got it. I discovered that my heater control valve is the culprit, and presto, my actuator opens. BUT I still get the air coming from defrost vents a speed and poor cooling as well.

I figured that the vacuum leak at the heater control valve would cover it, but is the vacuum reservoir still to blame? What about the poor cooling at speed? I'm a motorcycle guy, so my experience with AC is very limited...

 

Yes, the Vacuum reservoir stores vacuum for the Climate control and vent selector vacuum motors so they can work when engine intake has no vacuum, i.e. accelerating

No vacuum = Defrost vent selection, all vacuum motors off, default

There will be two hard plastic vacuum lines coming out of firewall in engine bay, passenger side
Grey line goes to Heat Control valve on Heater Hose
Black line is the main vacuum source and goes to Vacuum Reservoir

Hard plastic lines get brittle and break, also melt if they get too near exhaust pipes
They can be repair with correct size rubber lines, just it slide over the breaks

Follow the Black line to the reservoir
Remove it and then suck on it, it should hold vacuum, if not then there is a problem on that line or IN the CAB
If it holds vacuum then put line back on reservoir and follow reservoirs other line up to a "T" connector, or Check Valve, pull that line off and suck on it
If it doesn't hold vacuum then reservoir is cracked

It is possible that the check valve is bad, you should not be able to blow thru check valve
Then trace line to Vacuum Source, the intake manifold, remove that line and suck on it

 

Thanks for the info RonD

I pulled my splash shield off and tested the vacuum on the reservoir this last weekend and it seemed to hold, but I'll give it another whack following your instructions to the tee.

When you say "check valve" are you referring to the heater control valve? Otherwise I think you may be talking about where the source line come from the manifold