Classic/Antique Car Repair: 1930 cadillac v16, vacuum tanks, manifold vacuum

Question
i have a friend that has a cadillac with a v16 engine it has two vacuum thanks that feed the carbs with the problem is after it runs for a wile and gets hot ,you shut it off it dose not want to start,the after we put gas in each tank the car starts and runs fine, if shut off again the car may or may not start checked vacuum in on tank found to be aroune 6 lbs on fast ideal, any help offered?  Thanks

Answer
If the gauge you are using reads out in pounds, I don't know how to evaluate the reading - vacuum is measured in inches of mercury, abbreviated In. Hg.

Intake manifold vacuum at idle should be at least 17 In Hg - if it is only 6 In Hg, there is a problem with the engine.  It could have a serious vacuum leak, or be badly in need of a tune-up, or have a mechanical problem - which could be leaky valves or something more serious.

Vacuum tanks are quite tricky to service - you should get the car to a mechanic who is familiar with these to get them fixed if you have normal vacuum and they are not working right.