'65 Formula S Power Brake Booster Problem

About ten years ago I spent good money having the factory power brake booster renewed. Awhile ago I noticed diminished assist and at the same time the idle speed would change slightly when I depressed the brake pedal. Now it seems no power assist whatsoever and no change in idle speed when brake pressed. Well, pushing down hard on the brake pedal feels like it hardly moves at all.

Must be time to convert to a dual master cylinder anyway. I therefore ask for help/advice on steering me to the best youtube video or FABO thread on this site that will explain to me in simple terms what is involved in the conversion, the parts necessary, and parts source? Thank you for pointing me in the right direction.

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