1965 Valiant brake woes

I know this is not what you want to hear, but take out any brake part that does not match a single factory original brake system arrangement, and replace them with parts that do match. It sounds like what you are actually doing, but be sure to select a system that is dual 10” drums like you have, which arguably do not need a booster to work as drum brakes can lock up at 250 psi, where disc brakes should have a booster due to requiring 1500 psi or more to be effective. By making what you have match a factory designed system, and replacing any rubber parts in the process to make sure there is not a degraded collapsing brake hose or leaking wheel cylinder, you should have a brake system that works as good as the factory designed it to, provided that is good enough for you. For me I did power disc brake upgrade and picked 1973 Duster, no issues, excellent brakes. I know you are doing all drums, but it’s the same idea, factory put a lot of money into the design, why screw it up with non factory parts.