what would be the normal vacuum reading for a 1966 Ford 390 GT. Engine has been rebuilt, has Holley 4 barrel carb, looks rebuilt, all new hoses and a new power brake booster. The brake pedal (power brakes) is rock hard and stopping is a 2 foot high pressure chore. The vacuum now at idle is 11in/. There is no wild cam to drop the vacuum.
Somewhere between 15 and 22 . If its in good shape I would expect higher then 18. . late valve timing .... vacuum leak ..... I think I would plug off the new booster first to see what happens ? This may be of help
I would suspect somrthing wrong with the booster .Check engine vaccum on its own .sbould read around 17, 18 pounds.i would check the rod from the pedal to the booster under the dash to be sure it has proper clearance.
My 66 mercury 390 with a hot cam and tri power the most vacumm I can get from it is about 18.5 pounds and I set the timing to 17.5 pounds.
Start by checking to engine on its own .
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It' a good thing we dont get as much goverment as we pay for !!!