Vacuum is at its lowes when the throttle is wide open, and engine speed is low. At idle, or when closing the carb there is plenty of vacuum, but the check valve ensures that the vacuum reservoir is keept at peek vacuum, and it should hold enough for multiple stops (nice if the engine stalls). The valve type is not critical, and just about any type should do.
My 59 has no powerbrakes, and still the brakes are not hard to operate - so it should work resonably well even wothout powerassist.
The easy way to check the function of the power brakes is to stop the engine, press the brake pedal with normal operating pressure, start the engine and the pedal should immidiatly drop a noticeable amount.
First step a few times on the brake with the engine off to use any remaining vacuum though.