[srmmmm]

After finally getting a vacuum gauge hooked up, I found I can only generate 12 inches of vacuum@900rpm and thus I think the root cause of why my HVAC controls are stuck in Bi-Level mode. All diaphragms and vacuum switches are new and worked when tested individually. I also added a vacuum storage canister.

Does anyone have a recommendation for an auxiliary vacuum pump? I think that's going to be my only solution.

[Squidward]

On a related side note: are you running manifold or ported vacuum advance? Maybe switching might give you a few more inches at 900 rpm.

My 67 Firebird came from the factory with a tucked away reserve vacuum canister to crutch up for just such a thing. So it validates that the struggle is real.

[JLMounce]

Your profile states you're in the DFW area, so this may not be a concern, but you need to watch out with vacuum pumps anywhere near high altitude. If you do, or plan to take the car to somewhere like Colorado, you can damage the vacuum pump. Most of them are set on a static switch typically to provide around 18" of vacuum. They'll never reach that at higher altitudes where barometric pressure is lower, so they run constantly and burn up.

I would think that the vacuum canister alone should be all that is necessary as it will store the highest vacuum received, which is typically during light cruise. How much vacuum do you see in that driving situation? I'm not well versed on how much vacuum the diaphragms require to operate correctly, but I wouldn't think you'd need 18+ inches of Hg. Maybe I'm wrong? Is it possible the hvac door and hinge mechanisms need lubricated to make it easier for the diaphragms to work optimally?

[srmmmm]

I'm running manifold vacuum, and do have a canister in the circuit, but can't get anything to move. I am going to try to see if anything happens with a manual handheld vacuum pump first. If not, then I've got one more main vacuum control switch that I can install to try.

[srmmmm]

Good news and bad news with the handheld vacuum pump test. Good news is no leaks, system held 25 inches of vacuum throughout the test. Bad news is nothing moved. No change in any of the air door positions. Time to check the routing of the lines to the vacuum switch manifold. Glad I didn't spring for an auxiliary pump though.