power or not

As soon as your vacuum rises above about 12 to 14 inches it will work perfectly. If your check-valve works properly then the booster will work almost all the time.
There are a couple of situations where the booster might be iffy;
One is when you are backing out of the carport, with the an automatic, and the engine NOT on choke, after it has been sitting for a long time. But with that cam, you are probably running a pretty loose TC, so at least you won't have to fight engine torque. The solution for me, manual trans, was to just blip the throttle a time or two, to evacuate the chamber, before reversing. After that,I never gave the brakes another thought.
Two is sitting at a stoplight, in gear. It is possible for the vacuum in the booster, to fall below a threshold to provide assist, and then you have to compensate with leg-power. My application was with a manual trans and so I never ran into this. With an auto, just put it into neutral.
Three is a long braking distance with the engine idling below the minimum vacuum. Again with a stick and 11/1 compression, I never experienced this.
Four is multiple brake applications, with the engine at or near idle. This happens in bumper-to-bumper traffic, with the rpm down. For an automatic, this is the deal-breaker. But the good news is that you would be in the same situation with manual brakes,lol. And, you are now at a slow enough speed, that it shouldn't require a whole lot of leg compensation.
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Having said all that; the dual-diaphragm did not work for me. It requires a higher vacuum to stay working. At least mine did. I even took it apart to try and hot-rod it, but there was nothing I could do with it.
I ended up with a large single diaphragm unit from either a 73 Dart, or possibly a 77 Aspen.
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I love my power-brakes, with a 15/16 M/C. I adjusted my pushrod to the minimum length to keep the C-port open and this gave me just the right amount of assist, at all times. My engine, now with a much smaller cam, idles at 10/11 inches.It's a manual trans, and I can drag the engine rpm down from it's 700/750 idle to as low as 550 with a timing retarder. It will pull itself anywhere on a flat,hard,level surface, and the vacuum is way below boost threshold. But the instant after the rpm crosses about 1000/1200rpm/8mph, the booster is back on line.
I am using the KH 4piston calipers, and 10" rear drums with either 7/8s or 15/16 WCs (can't recall at the moment,lol). Works great for my application; street.