Since there is a vacuum reservoir, I would have thought that even if there were a significant vacuum leak that it would take at least a few seconds for the vacuum to drop to zero after turning the engine off, but it immediately drops to zero (I checked this with a vacuum gauge hooked up to to the vacuum lines).
I don't hear any vacuum leaking (although I don't have a vacuum pump to test with), so I don't understand why the vacuum drops so quickly.
I have replaced the vacuum check-valve and filter.
All of the vacuum operated controls seem to work fine with the engine running.
Any ideas of what to look for?

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Could the vacuum reservoir be leaking?  Mine is just a large tin can.

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1973 L-82 4 spd

This is a 75, which uses the tubler front frame piece as the reservoir.  I would think that a leak large enough to drain it in a second when the engine is shut off would be enough to keep the vacuum components from working even when the engine is running.




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My bad,I was thinking of the earlier reservoir. You could use a Mityvac to check the tank and sections of hose.

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1973 L-82 4 spd

The engine has to be running to get vacuum from the intake manifold. The fraction of a second you turn off  the engine it creates a negative pressure. If you have the car running with headlights on and forget to close them when you shut the car off they stay up. You need to restart engine to have vacuum to close them. That is how they operate. To get a better explanation I googled does a corvette engine need to be running for vacuum. Then scroll down to superchevy site. http://www.superchevy.com/how-to/vemp-0212-corvette-vacuum-systems-guide/

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The question is, WHERE did you have the gauge connected? Anywhere other than in the headlight circuit, the vac WILL go to 0 with the engine off. The headlights(and wiper door on early models) are the only components(other than the brake booster, which has it's own separate vac source & check valve) that retain vacuum with the engine off. The headlights should be able to operate for a short time after the engine is off from the vac reservoir.



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Joel Adams
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Ah, that makes a lot of sense.  I checked it just past the check valve off of the intake manifold.  I need to get a "Y" fitting so I can check it at the vacuum tank.
I didn't know there was more than one check valve.
Thanks!



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