The effect of 10 in of vacuum is most noticeable when multiple brake applications are being made. Such as rolling up to a stop light, with each peddle touch the brake peddle becomes harder to apply with less braking until you just about have to stand on the thing to stop. Get the vacuum up or remove the booster and go manual otherwise you are just gambling.

To demonstrate just go out get the car rolling 30mph, slip it into neutral cut the engine, try and stop. All power assist braking cars will respond the same. Oh yes leave enough room to start the motor or use the emergency brake.

On the canister thing, the canister has a one way check it is possibly to get above the 10 in mark. The reason is if you have a vacuum gage you will see that coming off the power the vacuum will go up. BUT quote Summit :These vacuum canisters capture an extra shot of vacuum from your engine for your power brakes" AN sounds like one and thats not going to give you repeated braking with the same force from the brakes. If you just go around the block to car shows, maybe a fit but is you drive your car for the enjoyment then you should consider a pump or revert to manual non assist brakes.

Me I want my brakes to do their job with the same efficiency every time all the time .

Dollars and sense buy pump: 350.00.
Replace with manual MC: 150.00 + tossing the booster ( 7" DUAL DIAPHRAGM POWER BOOSTER $195.00) total for manual conversion 345.00?