The vacuum assist (booster) makes no difference. If it worked with the drum brakes, it will work with the discs.
When swapping to disc fronts, it's best to use the '69-'72 A-body disc/drum master cylinder. Don't forget to also pull the prop valve from the Buick-on-Chevy brake system and put it on the LeMans.
On another note, you can swap out boosters to the '78-'87 10 inch dual diaphragm booster. Better response and pedal feel.
Check my web site here.
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Freedom is never more than one generation away from extinction. We didn't pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same.
-RONALD REAGAN
462 cid/ 6x-4/ TH400/ 3.36:1/ 28x10.5/ 3880#/ 12.35 @ 109.36/ 1.69 60 ft/ 4 wheel disc brakes/ 15 mpg