That is not the stock booster, most likely a 9" or 10" single aftermarket. Do the math with surface area vs. 11" and you will answer your own question.
Stock 11" single diaphragm are good to about 11 inHg, single 9/10's like the one shown are mid-teens; hence his issue.
Dual 9" work at least to 9 inHg from personal experience.
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