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Re: A/C posessed!
Hi,
You can simply add the check valve to the existing line, leaving the old checkvalve in the system, if you can't locate it. Start off by finding the manifold vacuum port on the engine, in 1978 if I remember correctly there are a myriad of vacuum hoses strung around the engine to thermal port switches and such mostly for emission controls. The hose you'll be searching for leads back to the firewall, it will be of similar size to that of the vacuum hose for the transmission modulator. Once you found that hose observe the flow arrow on your new checkvalve (might have to refer to packaging for diagram) if not clearly labeled. The one I bought had two different colors denoting the flow direction.
You'll want to place the checkvalve in line with this vacuum hose with the flow or arrow pointing toward the engine.
That should cure that, if not, then you have a vacuum leak in the HVAC control hoses, reservoir (if the car has one), dash selector, or door diaphram servos. You'll need a small hand vacuum pump and gauge (cheap one sells at autozone, or napa) then isolate individual components and test.
Hope this helps,
Keith
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