My '75 has a vacuum leak somewhere in the headlight circuit (I isolated the heater controls and these are fine).  I managed to borrow a vacuum pump to see if I could hear where the leak was coming from, but I guess the leak is so bad that I could not get any vacuum up at all.  I tried a hose to my ear to each connection to see if I could hear the leak, but nothing (an open circuit won't make much noise. . .).

I'd like to test each section of the vacuum lines to see if I can narrow it down, but this is a confusing array of hoses and controls!

Does someone have a vacuum diagram, and possibly some pointers on which hoses can be isolated to test each area of the system?

The headlights do open and close okay, but the system does not hold any vacuum once the engine is off.  I have replaced the check valve and filter at the carb.

I'd appreciate any help you can provide!

Have you isolated the holding tank at the front of the car? It's the large horizontal tube just under the front edge of the hood. If it leaks, problem found. It may hold enough vacuum while engine is running to operate lights, but not while stopped.

|UPDATED|3/21/2017 5:53:35 PM (AZT)|/UPDATED|

Yes, I do know where this is.  I'll apply vacuum to it in the morning to see if that holds okay.

I really need a diagram so I can figure out how to isolate specific parts of the vacuum system to test each by themselves.  I think that will be the most efficient way to track this leak down.
Thanks.

One of the major aftermarket Corvette suppliers sells an vacuum hose diagram.  Just can't remember which one.

Here is a link to you tube from willcox which demostrates how to troubleshoot this problem !

That was a helpful video that clearly explained each component.  Thanks for the link!

Well, after testing section by section, I don't believe there one big leak, but many small ones.  Since everything works and the only issue I was trying to address was that the system would not hold vacuum as soon as the engine was turned off (I like everything to work as designed), I'm giving up and living with it as is.
Thanks for everyone's input.

Time for a vacuum hose replacement kit?   

|UPDATED|3/22/2017 6:42:14 PM (AZT)|/UPDATED|

Yea, that's on my wish list, but not ready to dive into it yet. . .

If you go on Eckler Corvettes site and look up the vacuum hose kit they have a PDF that you can open and it shows the complete system from the head lite switch all the way to the front