'65 Formula S Power Brake Booster Problem

Agree with the above, make SURE there's no leakage at the tank, hoses, or valve.
If you do go the replacement route, check Doctor Diff's offerings. He's one of the only sources for a Mopar-specific booster right now (no tank needed), and also has the brackets and MCs to go with it.
Bendix Style Dual Diaphragm Booster for Mopars
(Listed for '67-'76, but will work on your '65)