I installed a Compcams vacuum pump and it provides 22 inches of vacuum all of the time.
The vacuum booster in the car actually raises the amount of pressure that the brake calipers see. Approximately 200 psi at the caliper more.
Without boost they see about 500 psi. With boost about 700. That is where the assist is.
Every brand of brake booster varies with the amount of additional pressure it provides and the minimum amount of vacuum it needs to work. They all have their own idiosyncrasies.
I found that the Pantera booster will provide almost no boost with less then 12 inches of vacuum and even at 14 inches, the amount of boost is marginal and very little. I presume, without having access to the original manufacturers design specification/design sheet, that therefore there is an intended design requirement of about 17-18 inches with a plus/minus of about 4 inches.
I tested this on the car with a pressure gauge in the calipers to measure what the caliper was getting. I could not vary the amount of vacuum the booster provides, otherwise I could have written a spec chart of all the variations.
When I installed the Webers the most I could get at idle was about 14 inches and when I went to a longer duration cam, it dropped to 12. So the vacuum pump was the simplest solution. It makes the brake pedal feel consistent with little or no variation.
The Compcams version is noisy but in the engine compartment near the headers it doesn't matter much.
Others have been successful in adapting the Volvo vacuum booster from the turbo diesel equipped vehicles that they claim is quiet.