Are you losing fluid?
If not then the Booster is likely your issue, check your vacuum/connections.
I have the factory power drum single reservoir on my '66 and keep it all in good order with no issues during hard braking,
Yes the dual res is a good idea but not really necessary if all is in order.
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When I die, I want to go peacefully like my grandfather did, in his sleep.
Not screaming like the passengers in his car. 
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