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JB Weld should work fine for that. Just start it up and listen for a leak. Mark the spots where you hear the leaks. Shut off the engine and apply the JB Weld. Let it dry for a couple of hours and repeat the process. Keep doing until you have them all plugged. Then you can sand the JB weld smooth and spray with a little black paint. Good as new.
The only vacuum leak I have found on my car was a broken plastic T-fitting. I couldn't find an exact replacement so I tried JB Weld to glue it back together. It has held for almost a year.
For a vacuum tank, you put the repair on the OUTSIDE...not the inside. I agree with above, use JB Weld. Or you could lay some fiberglass and resin over the leak areas (on the outside).