'65 Formula S Power Brake Booster Problem

You might have a leak in the tank, booster or the check valve on the booster might be bad. I would check them first. Swapping to a dual MC might not have any effect if these other components are defective

I am not that familiar with the early booster so I can’t say with any authority that you can just bolt on s dual MC and go.

Nevertheless, converting to a modern dual MC is a upgrade, especially in terms of safety