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My '75 F350 donor dual diaphragm booster/brackets had the large 1-1/4" bore MC on it at the time I got the brake booster from the wrecking yard.
... etc ...
The 1-1/4" or 1-1/16" MC bore MIGHT have been dependent on whether the booster installed was a direct connection to the brake pedal or, whether it was mounted to a bellcrank bracket assembly (?).
Now that my truck has front discs and a slight reduction in output pressure of the slightly larger bore MC (from the previous 1.00" bore to the current 1-1/16" bore), the brake balance is very good.
That all seems to jive with what I found .... the '77 used the bellcrank bracket assy.
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