vacuum booster?

[familyfast64]

Hey guys, forgive me but I'm not real familiar with these cheaper black summit or mines jegs vacuum assist cans. Mine has a gauge that I can read but I don't know if that's ok to use when adjusting your fuel air screws on the carb for best vacuum or if I should continue using the vacuum gauge that I hook to the carb? Also, wouldn't the vacuum can effect your vacuum secondaries? Does it not effect the engine at all, just the brakes?
I just thought I would ask for a rundown on how they work and if carb tuning and vacuums secondaries are effected by them. Thanks.